Programming C# 4.0 SIXTH EDITION Programming C# 4.0 Ian Griffiths, Matthew Adams, and Jesse Liberty Beijing • Cambridge • Farnham • Köln • Sebastopol • Taipei • Tokyo Programming C# 4.0, Sixth Edition by Ian Griffiths, Matthew Adams, and Jesse Liberty Copyright © 2010 Ian Griffiths and Matthew Adams. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://my.safaribooksonline.com). For more information, contact our corporate/institutional sales department: 800-998-9938 or corporate@oreilly.com. Editors: Mike Hendrickson and Laurel Ruma Production Editor: Adam Zaremba Copyeditor: Audrey Doyle Proofreader: Stacie Arellano Indexer: Jay Marchand Cover Designer: Karen Montgomery Interior Designer: David Futato Illustrator: Robert Romano Printing History: July 2001: First Edition. February 2002: Second Edition. May 2003: Third Edition. February 2005: Fourth Edition. December 2007: Fifth Edition. August 2010: Sixth Edition. Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. Programming C# 4.0, the image of an African crowned crane, and related trade dress are trademarks of O’Reilly Media, Inc. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and O’Reilly Media, Inc. was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information con- tained herein. ISBN: 978-0-596-15983-2 [M] 1280338225 Table of Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv 1. Introducing C# .......................................................... 1 Why C#? Why .NET? 1 The .NET Framework Class Library 2 Language Style 3 Composability 4 Managed Code 5 Continuity and the Windows Ecosystem 6 C# 4.0, .NET 4, and Visual Studio 2010 7 Summary 9 2. Basic Programming Techniques . . . ....................................... 11 Getting Started 11 Namespaces and Types 14 Projects and Solutions 19 Comments, Regions, and Readability 24 Bad Comments 26 XML Documentation Comments 26 Variables 28 Variable Types 28 Expressions and Statements 35 Assignment Statements 38 Increment and Decrement Operators 38 Flow Control with Selection Statements 39 if Statements 40 switch and case Statements 45 Iteration Statements 47 foreach Statements 48 for Statements 50 while and do Statements 52 v Breaking Out of a Loop 53 Methods 55 Summary 58 3. Abstracting Ideas with Classes and Structs ................................. 59 Divide and Conquer 59 Abstracting Ideas with Methods 59 Abstracting Ideas with Objects and Classes 62 Defining Classes 64 Representing State with Properties 64 Protection Levels 66 Initializing with a Constructor 68 Fields: A Place to Put Data 72 Fields Can Be Fickle, but const Is Forever 75 Read-only Fields and Properties 76 Related Constants with enum 79 Value Types and Reference Types 82 Too Many Constructors, Mr. Mozart 88 Overloading 88 Overloaded Methods and Default Named Parameters 89 Object Initializers 92 Defining Methods 95 Declaring Static Methods 98 Static Fields and Properties 99 Static Constructors 101 Summary 102 4. Extensibility and Polymorphism . . . ...................................... 103 Association Through Composition and Aggregation 104 Inheritance and Polymorphism 106 Replacing Methods in Derived Classes 109 Hiding Base Members with new 109 Replacing Methods with virtual and override 112 Inheritance and Protection 114 Calling Base Class Methods 116 Thus Far and No Farther: sealed 118 Requiring Overrides with abstract 121 All Types Are Derived from Object 127 Boxing and Unboxing Value Types 127 C# Does Not Support Multiple Inheritance of Implementation 132 C# Supports Multiple Inheritance of Interface 132 Deriving Interfaces from Other Interfaces 135 Explicit Interface Implementation 136 vi | Table of Contents The Last Resort: Checking Types at Runtime 141 Summary 142 5. Composability and Extensibility with Delegates .... . . . . . . . . . . . . . . . . . . . . . . . . 143 Functional Composition with delegate 150 Generic Actions with Action 156 Generic Predicates with Predicate 160 Using Anonymous Methods 162 Creating Delegates with Lambda Expressions 163 Delegates in Properties 165 Generic Delegates for Functions 167 Notifying Clients with Events 171 Exposing Large Numbers of Events 180 Summary 183 6. Dealing with Errors . . . ................................................. 185 When and How to Fail 191 Returning Error Values 194 Debugging with Return Values 200 Exceptions 201 Handling Exceptions 207 When Do finally Blocks Run? 214 Deciding What to Catch 215 Custom Exceptions 218 Summary 220 7. Arrays and Lists . . . .................................................... 221 Arrays 221 Construction and Initialization 222 Custom Types in Arrays 225 Array Members 230 Array Size 236 List 243 Custom Indexers 247 Finding and Sorting 253 Collections and Polymorphism 254 Creating Your Own IEnumerable 258 Summary 264 8. LINQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Query Expressions 265 Query Expressions Versus Method Calls 267 Extension Methods and LINQ 268 Table of Contents | vii let Clauses 271 LINQ Concepts and Techniques 271 Delegates and Lambdas 271 Functional Style and Composition 273 Deferred Execution 274 LINQ Operators 275 Filtering 275 Ordering 276 Concatenation 279 Grouping 280 Projections 282 Zipping 288 Getting Selective 289 Testing the Whole Collection 291 Aggregation 292 Set Operations 294 Joining 295 Conversions 296 Summary 297 9. Collection Classes ... .................................................. 299 Dictionaries 299 Common Dictionary Uses 301 IDictionary 308 Dictionaries and LINQ 309 HashSet and SortedSet 310 Queues 311 Linked Lists 312 Stacks 313 Summary 314 10. Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 What Is a String? 316 The String and Char Types 317 Literal Strings and Chars 318 Escaping Special Characters 319 Formatting Data for Output 322 Standard Numeric Format Strings 323 Custom Numeric Format Strings 329 Dates and Times 332 Going the Other Way: Converting Strings to Other Types 336 Composite Formatting with String.Format 337 Culture Sensitivity 338 viii | Table of Contents Exploring Formatting Rules 340 Accessing Characters by Index 341 Strings Are Immutable 341 Getting a Range of Characters 343 Composing Strings 344 Splitting It Up Again 346 Upper- and Lowercase 347 Manipulating Text 348 Mutable Strings with StringBuilder 349 Finding and Replacing Content 353 All Sorts of “Empty” Strings 355 Trimming Whitespace 357 Checking Character Types 360 Encoding Characters 360 Why Encodings Matter 362 Encoding and Decoding 363 Why Represent Strings As Byte Sequences? 370 Summary 370 11. Files and Streams ... .................................................. 371 Inspecting Directories and Files 371 Examining Directories 374 Manipulating File Paths 375 Path and the Current Working Directory 376 Examining File Information 377 Creating Temporary Files 381 Deleting Files 381 Well-Known Folders 383 Concatenating Path Elements Safely 387 Creating and Securing Directory Hierarchies 388 Deleting a Directory 394 Writing Text Files 396 Writing a Whole Text File at Once 396 Writing Text with a StreamWriter 397 When Files Go Bad: Dealing with Exceptions 400 Finding and Modifying Permissions 404 Reading Files into Memory 409 Streams 413 Moving Around in a Stream 419 Writing Data with Streams 421 Reading, Writing, and Locking Files 422 FileStream Constructors 423 Stream Buffers 423 Table of Contents | ix Setting Permissions During Construction 424 Setting Advanced Options 425 Asynchronous File Operations 425 Isolated Storage 428 Stores 429 Reading and Writing Text 430 Defining “Isolated” 431 Managing User Storage with Quotas 436 Managing Isolated Storage 436 Streams That Aren’t Files 439 An Adapting Stream: CryptoStream 443 In Memory Alone: The MemoryStream 444 Representing Binary As Text with Base64 Encoding 444 Summary 447 12. XML ................................................................ 449 XML Basics (A Quick Review) 449 Elements 450 XHTML 451 X Stands for eXtensible 452 Creating XML Documents 452 XML Elements 455 XML Attributes 456 Putting the LINQ in LINQ to XML 459 Searching in XML with LINQ 461 Searching for a Single Node 465 Search Axes 466 Where Clauses 466 XML Serialization 467 Customizing XML Serialization Using Attributes 469 Summary 471 13. Networking .......................................................... 473 Choosing a Networking Technology 473 Web Application with Client-Side Code 474 .NET Client and .NET Server 477 .NET Client and External Party Web Service 479 External Client and .NET Web Service 480 WCF 481 Creating a WCF Project 481 WCF Contracts 482 WCF Test Client and Host 483 Hosting a WCF Service 486 x | Table of Contents Writing a WCF Client 493 Bidirectional Communication with Duplex Contracts 501 HTTP 511 WebClient 512 WebRequest and WebResponse 516 Sockets 522 IP, IPv6, and TCP 523 Connecting to Services with the Socket Class 528 Implementing Services with the Socket Class 531 Other Networking Features 536 Summary 537 14. Databases ........................................................... 539 The .NET Data Access Landscape 539 Classic ADO.NET 540 LINQ and Databases 544 Non-Microsoft Data Access Technologies 545 WCF Data Services 546 Silverlight and Data Access 546 Databases 547 The Entity Data Model 548 Generated Code 551 Changing the Mapping 554 Relationships 555 Inheritance 562 Queries 563 LINQ to Entities 563 Entity SQL 568 Mixing ESQL and LINQ 570 The EntityClient ADO.NET Provider 571 Object Context 571 Connection Handling 571 Creating, Updating, and Deleting 574 Transactions 576 Optimistic Concurrency 581 Context and Entity Lifetime 583 WCF Data Services 584 Summary 588 15. Assemblies .......................................................... 589 .NET Components: Assemblies 589 References 590 Writing Libraries 593 Table of Contents | xi Protection 595 Naming 598 Signing and Strong Names 599 Loading 601 Loading from the Application Folder 602 Loading from the GAC 603 Loading from a Silverlight .xap File 603 Explicit Loading 604 Summary 605 16. Threads and Asynchronous Code ......................................... 607 Threads 609 Threads and the OS Scheduler 611 The Stack 613 The Thread Pool 620 Thread Affinity and Context 622 Common Thread Misconceptions 623 Multithreaded Coding Is Hard 629 Multithreading Survival Strategies 632 Synchronization Primitives 634 Monitor 634 Other Lock Types 645 Other Coordination Mechanisms 649 Events 649 Countdown 650 BlockingCollection 650 Asynchronous Programming 651 The Asynchronous Programming Model 652 The Event-Based Asynchronous Pattern 655 Ad Hoc Asynchrony 656 The Task Parallel Library 656 Tasks 657 Cancellation 663 Error Handling 665 Data Parallelism 666 Parallel For and ForEach 667 PLINQ: Parallel LINQ 669 Summary 670 17. Attributes and Reflection . . . ........................................... 671 Attributes 671 Types of Attributes 672 Custom Attributes 673 xii | Table of Contents Reflection 677 Inspecting Metadata 678 Type Discovery 679 Reflecting on a Specific Type 681 Late Binding 683 Summary 686 18. Dynamic ............................................................. 687 Static Versus Dynamic 687 The Dynamic Style and COM Automation 689 The dynamic Type 690 Object Types and dynamic 693 dynamic in Noninterop Scenarios? 703 Summary 706 19. Interop with COM and Win32 ........................................... 707 Importing ActiveX Controls 707 Importing a Control in .NET 708 Interop Assemblies 711 No PIA 712 64-bit Versus 32-bit 713 P/Invoke 716 Pointers 720 C# 4.0 Interop Syntax Enhancements 725 Indexed Properties 725 Optional ref 726 Summary 727 20. WPF and Silverlight . . . ................................................ 729 Xaml and Code Behind 731 Xaml and Objects 735 Elements and Controls 738 Layout Panels 739 Graphical Elements 748 Controls 755 User Controls 760 Control Templates 761 Styles 764 The Visual State Manager 766 Data Binding 767 Data Templates 769 Summary 773 Table of Contents | xiii 21. Programming ASP.NET Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775 Web Forms Fundamentals 775 Web Forms Events 776 Web Forms Life Cycle 778 Creating a Web Application 779 Code-Behind Files 780 Adding Controls 781 Server Controls 783 Data Binding 784 Examining the Code 789 Adding Controls and Events 790 Summary 794 22. Windows Forms . . . ................................................... 795 Creating the Application 796 Adding a Binding Source 797 Controls 800 Docking and Anchoring 805 Data Binding 806 Event Handling 811 Summary 813 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 xiv | Table of Contents Preface Microsoft unveiled the .NET Framework in 2000, and in the decade that followed, it became an extremely popular choice for developing software for Windows. While .NET supports many programming languages, it is most strongly associated with the language designed specifically for the platform: C#. C# has grown considerably since its launch. Each new version enabled new program- ming techniques—C# 2.0 added generics and enhanced functional programming ca- pabilities, then integrated query features and yet more powerful functional capabilities arrived in C# 3.0, and now C# 4.0 adds new dynamic language capabilities. The .NET Framework has grown with the language. Back in .NET 1.0, the class libraries offered relatively patchy coverage of the underlying Windows capabilities. Moreover, the library features that were unique to .NET, rather than being wrappers for something else, were relatively modest. Now, as well as more comprehensive platform coverage we have a GUI framework (WPF), much stronger database capabilities, powerful sup- port for concurrent execution, and an extensive set of communication services (WCF), to name just a few of the available features. And the features that have been there since version 1.0, such as web support (ASP.NET), have been fleshed out substantially. .NET is no longer limited to running just on Windows. Some people recognized its potential for platform independence early on, but for years, Microsoft supported C# just on Windows, leaving open source projects to offer the only way to run C# on other systems. But in 2008, the release of Silverlight 2 saw C# code running with Microsoft’s full support on non-Windows platforms such as the Mac for the first time. The C# language has come a long way since 2000, in both reach and size. Our goal with Programming C# 4.0 is to show how to use C#. How This Book Is Organized The book begins by looking at the details of the C# language that you will use in everyday programming. We then look at the most common parts of the .NET Frame- work class library that you will also use very regularly. Next, we move into some more xv specialized areas of the framework. Finally, we look at some of the application frame- works for building Windows and web applications in .NET. Chapter 1, Introducing C# This chapter talks about the nature of C# and its relationship with the .NET Framework. Chapter 2, Basic Programming Techniques In this chapter, we show the core elements of C# code—the steps required to get up and running, and fundamental features such as variables, flow control, loops, and methods. Chapter 3, Abstracting Ideas with Classes and Structs C# supports object-oriented programming, and this chapter describes the lan- guage features dedicated to these techniques. Chapter 4, Extensibility and Polymorphism This chapter continues the discussion from the preceding chapter, illustrating how C# supports inheritance, interfaces, and related concepts. Chapter 5, Composability and Extensibility with Delegates C# isn’t limited to object-oriented programming—it also supports some very powerful functional programming idioms. This chapter shows how these can sometimes be more flexible and also simpler than OO techniques. Chapter 6, Dealing with Errors All programs encounter failures, whether due to programming errors, unexpected input, network failures, or a host of other eventualities. This chapter shows the options for detecting and responding robustly to errors. Chapter 7, Arrays and Lists This chapter shows the tools C# offers for representing simple collections of information. Chapter 8, LINQ It’s not enough merely to be able to represent collections, so this chapter shows how you can use the integrated query features in C# to process your collections of data. Chapter 9, Collection Classes This chapter shows some of the more specialized classes for working with collec- tions in particular ways. Chapter 10, Strings Text is a particularly important data type for most applications, so this chapter shows how text is represented, and how you can format data into textual form. Chapter 11, Files and Streams This chapter shows how to store information on disk and read it back in, and how to perform other filesystem operations. It also shows how some of the abstractions used when working with files can be applied in other scenarios. xvi | Preface Chapter 12, XML This chapter shows the classes offered by the .NET Framework for processing XML, and how these can work in conjunction with the LINQ features in C#. Chapter 13, Networking In this chapter, we look at the various techniques for communicating over a network. Chapter 14, Databases This chapter shows how to access a database from C#. Chapter 15, Assemblies In this chapter, we show how to compile code into libraries for reuse, and how programs made up from multiple components work. Chapter 16, Threads and Asynchronous Code Many programs need to deal with concurrency, and this chapter shows the tools and techniques available. Chapter 17, Attributes and Reflection C# has the ability to inspect the structure of code, which makes it easier to auto- mate certain kinds of tasks. This chapter shows the API for doing this, and how you can extend the structural information through attributes. Chapter 18, Dynamic One of the new features in C# 4.0 is support for dynamic binding. This is partic- ularly useful in certain interop scenarios, as we discuss in this chapter. Chapter 19, Interop with COM and Win32 Sometimes it’s necessary for C# code to communicate with components not de- signed to be used from .NET. This chapter shows how to do this with both COM components and Win32-style DLLs. Chapter 20, WPF and Silverlight WPF and Silverlight offer very similar programming models for building user in- terfaces. This chapter shows how to use that model from C#. Chapter 21, Programming ASP.NET Applications This chapter shows how to use ASP.NET, the part of the .NET Framework de- signed for building web applications. Chapter 22, Windows Forms This chapter shows how to use Windows Forms, which is a wrapper around the classic Windows user interface mechanisms. While it is less flexible than WPF, it can offer an easier way to integrate with old components such as ActiveX controls. Where to Find Features New in C# 4.0 and .NET 4 Although this book is written to be read as a whole, we expect that some readers will want to look for the features new to C# 4.0, and also to .NET 4. Since our goal is to show how the C# language is used today, we have avoided structuring the book around Preface | xvii the history of the language, because you will use language features of varying ages in combination. As it happens, one of the new features in C# 4.0 serves a very specific purpose, so it gets its own chapter, but for the most part, new language features are spread throughout the book, because we aim to mention them where you need to know about them. We cannot point you at a particular set of chapters, so instead, here’s a quick guide to where we discuss these features. Chapter 1 talks about the broad goals behind the new features in C# 4.0. Chapter 3 shows the use of default values and named arguments (and these come up again very briefly in Chapters 11 and 17). Chapter 7 describes variance, a rather technical feature of the type system that has some useful implications for collection types. Chapter 16 talks about the extensive new multithreading support added in .NET 4. Chapter 18 is dedicated entirely to a new language feature: support for dynamic programming. Chapter 19 describes the new no-PIA feature, and some features that allow more elegant code in some interop scenarios. Who This Book Is For If you have some basic knowledge of C# but want to brush up your skills, or if you are proficient in another programming language such as C++ or Java, or even if C# is your first programming language, this book is for you. What You Need to Use This Book To make the best use of this book, please obtain the latest release of Visual Studio 2010. Any edition will do, including the free Express edition for C#, which can be downloaded from http://www.microsoft.com/express/. For Chapter 14 you will need a copy of SQL Server or SQL Server Express. Some editions of Visual Studio will install SQL Server Express for you by default, so you may already have this. The example source code for this book is available through the O’Reilly site at http:// oreilly.com/catalog/9780596159832/. Conventions Used in This Book The following font conventions are used in this book: Italic is used for: • Pathnames, filenames, and program names • Internet addresses, such as domain names and URLs • New terms where they are defined xviii | Preface Constant Width is used for: • Command lines and options that should be typed verbatim • Names and keywords in program examples, including method names, variable names, and class names Constant Width Italic is used for: • Replaceable items, such as variables or optional elements, within syntax lines or code Constant Width Bold is used for: • Emphasis within program code Pay special attention to notes set apart from the text with the following icons: This is a tip. It contains useful supplementary information about the topic at hand. This is a warning. It helps you solve and avoid annoying problems. Using Code Examples This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: "Programming C# 4.0, Sixth Edition, by Ian Griffiths, Matthew Adams, and Jesse Liberty. Copyright 2010 Ian Griffiths and Matthew Adams, 978-0-596-15983-2.” Preface | xix Safari® Books Online Safari Books Online is an on-demand digital library that lets you easily search over 7,500 technology and creative reference books and videos to find the answers you need quickly. With a subscription, you can read any page and watch any video from our library online. Read books on your cell phone and mobile devices. Access new titles before they are available for print, and get exclusive access to manuscripts in development and post feedback for the authors. Copy and paste code samples, organize your favorites, down- load chapters, bookmark key sections, create notes, print out pages, and benefit from tons of other time-saving features. O’Reilly Media has uploaded this book to the Safari Books Online service. To have full digital access to this book and others on similar topics from O’Reilly and other pub- lishers, sign up for free at http://my.safaribooksonline.com/?portal=oreilly. Acknowledgments From Ian Griffiths I want to thank the technical reviewers, whose feedback helped to improve this book: Nicholas Paldino, Chris Smith, Chris Williams, Michael Eaton, Brian Peek, and Stephen Toub. Everyone at O’Reilly has provided a great deal of support and patience throughout the project, so many thanks to Mike Hendrickson, Laurel Ruma, Audrey Doyle, and Sumita Mukherji. Thanks also to John Osborn for getting things started in the early days of this project, and for getting Matthew and me on board as O’Reilly authors in the first place, all those years ago. Thank you to my coauthor for not learning his lesson from the last book and agreeing to write another with me. And finally, thank you to Jesse Liberty for asking us to take over his book. From Matthew Adams I’d like to add my thanks to those of my coauthor to all those at O’Reilly whose patience, help, and support have made this book possible, and to all our reviewers whose feed- back has been invaluable. In addition, I’d like to add a nod to Karolina Lemiesz, coffee wizard at the Starbucks where most of my text was written, for the constant supply of ristretto, and an education in coffee tasting when work got too much. xx | Preface As always, my partner Una provided the necessary foundation of love and support (despite her own book deadlines). And finally, anyone who tells you that squeezing a book out of an author is a breeze is clearly deluded, but my coauthor makes it look easy. My thanks go to him especially for his forbearance, wit, and friendship. And good dinners. Preface | xxi CHAPTER 1 Introducing C# C#—pronounced “See Sharp”—is a programming language designed for Micro- soft’s .NET platform. Since its first release in 2002, C# has found many roles. It is widely used on the server side of websites, and also on both the client and server in line-of-business Windows desktop applications. You can write smartphone user inter- faces and Xbox 360 games in C#. More recently, Microsoft’s Silverlight platform has made C# an option for writing Rich Internet Applications that run in a web browser. But what kind of language is C#? To understand a language well enough to use it effectively, it’s not enough to focus purely on the details and mechanisms, although we’ll be spending plenty of time on those in this book. It is equally important to un- derstand the thinking behind the details. So in this chapter, we’ll look at what problems C# was built to solve. Then we’ll explore the style of the language, through aspects that distinguish it from other languages. And we’ll finish the chapter with a look at the latest step in the evolution of C#, its fourth version. Why C#? Why .NET? Programming languages exist to help developers be more productive. Many successful languages simplify or automate tedious tasks that previously had to be done by hand. Some offer new techniques that allow old problems to be tackled more effectively, or on a larger scale than before. How much difference C# can make to you will depend on your programming background, of course, so it’s worth considering what sorts of people the language designers had in mind when they created C#. C# is aimed at developers working on the Windows platform, and its syntax is instantly familiar to users of C or C++, or other languages that draw from the same tradition, such as JavaScript and Java. Fundamental language elements such as statements, ex- pressions, function declarations, and flow control are modeled as closely as possible on their equivalents in C family languages. 1 A familiar syntax is not enough of a reason to pick a language, of course, so C# offers productivity-enhancing features not found in some of its predecessors. Garbage col- lection frees developers from the tyranny of common memory management problems such as memory leaks and circular references. Verifiable type safety of compiled code rules out a wide range of bugs and potential security flaws. While C or C++ Windows developers may not be accustomed to those features, they will seem old hat to Java veterans, but Java has nothing to compete with the “LINQ” features C# offers for working with collections of information, whether in object models, XML documents, or databases. Integrating code from external components is remarkably painless, even those written in other languages. C# also incorporates support for functional pro- gramming, a powerful feature previously most commonly seen in academic languages. Many of the most useful features available to C# developers come from the .NET Framework, which provides the runtime environment and libraries for C#, and all other .NET languages, such as VB.NET. C# was designed for .NET, and one of the main benefits of its close relationship with the .NET Framework is that working with framework features such as the class library feels very natural. The .NET Framework Class Library Working in C# means more than using just the language—the classes offered by the .NET Framework are an extremely important part of the C# developer’s everyday experience (and they account for a lot of this book’s content). Most of the library func- tionality falls into one of three categories: utility features written in .NET, wrappers around Windows functionality, and frameworks. The first group comprises utility types such as dictionaries, lists, and other collection classes, as well as string manipulation facilities such as a regular expression engine. There are also features that operate on a slightly larger scale, such as the object models for representing XML documents. Some library features are wrappers around underlying OS functionality. For example, there are classes for accessing the filesystem, and for using network features such as sockets. And there are classes for writing output to the console, which we can illustrate with the obligatory first example of any programming language book, shown in Example 1-1. Example 1-1. The inevitable “Hello, world” example class Program { static void Main() { System.Console.WriteLine("Hello, world"); } } 2 | Chapter 1: Introducing C# We’ll examine all the pieces shown here in due course, but for now, note that even this simplest of examples depends on a class from the library—the System.Console class in this case—to do its job. Finally, the class library offers whole frameworks to support building certain kinds of applications. For example, Windows Presentation Foundation (WPF) is a framework for building Windows desktop software; ASP.NET (which is not an acronym, despite appearances) is a framework for building web applications. Not all frameworks are about user interfaces—Windows Communication Foundation (WCF) is designed for building services accessed over the network by other computer systems, for instance. These three categories are not strict, as quite a few classes fit into two. For example, the parts of the class library that provide access to the filesystem are not just thin wrap- pers around existing Win32 APIs. They add new object-oriented abstractions, provid- ing significant functionality beyond the basic file I/O services, so these types fit into both the first and second categories. Likewise, frameworks usually need to integrate with underlying services to some extent—for example, although the Windows Forms UI framework has a distinctive API of its own, a lot of the underlying functionality is provided by Win32 components. So the three categories here are not strict. They just offer a useful idea of what sorts of things you can find in the class libraries. Language Style C# is not the only language that runs on the .NET Framework. Indeed, support for multiple languages has always been a key feature of .NET, reflected in the name of its runtime engine, the CLR or Common Language Runtime. As this name implies, .NET is not just for one language—numerous languages have access to the services of the .NET Framework class library. Why might you choose C# over the others? We already mentioned one important reason: C# was designed specifically for .NET. If you are working with .NET technologies such as WPF or ASP.NET, you’ll be speaking their language if you work in C#. Compare this with C++, which supports .NET through extensions to the original language. The extensions are carefully thought out and work well, but code that uses .NET libraries just looks different from normal C++, so programs that bridge the worlds of .NET and standard C++ never feel com- pletely coherent. And the dual personality often presents dilemmas—should you use standard C++ collection classes or the ones in the .NET class library, for example? In native .NET languages such as C#, such questions do not emerge. But C# is not unique in this respect. Visual Studio 2010 ships with three languages designed for .NET: C#, VB.NET, and F#. (Although VB.NET follows on from its non-.NET Visual Basic predecessors, it was radically different in some important ways. It is a native .NET language with a VB-like syntax rather than VB 6 with .NET capa- bilities bolted on.) The choice between these languages comes down to what style of language you prefer. Language Style | 3 F# is the odd one out here. It’s a functional programming language, heavily influenced by a language called ML. Back in 1991, when your authors were first-year students, our university’s computer science course chose ML for the first programming language lectures in part because it was so academic that none of the students would previously have come across anything like it. F# is still at the academic end of the spectrum despite having climbed far enough down the ivory tower to be a standard part of a mainstream development environment. It excels at complicated calculations and algorithms, and has some characteristics that can help with parallel execution. However, as with many functional languages, the cost of making some hard problems easier is that a lot of things that are easy in more traditional languages are remarkably hard in F#— functional languages are adept at complex problems, but can be clumsy with simple ones. It seems likely that F# will mostly be used in scientific or financial applications where the complexity of the computation to be performed dwarfs the complexity of the code that needs to act on the results of those calculations. While F# feels distinctly other, VB.NET and C# have a lot of similarities. The most obvious factor in choosing between these is that VB.NET is easier to learn for someone familiar with Visual Basic syntax, while C# will be easier for someone familiar with a C-like language. However, there is a subtler difference in language philosophy that goes beyond the syntax. Composability A consistent theme in the design of the C# programming language is that its creators tend to prefer general-purpose features over specialized ones. The most obvious ex- ample of this is LINQ, the Language INtegrated Query feature added in C# 3.0. Su- perficially, this appears to add SQL-like query features to the language, providing a natural way to integrate database access into your code. Example 1-2 shows a simple query. Example 1-2. Data access with LINQ var californianAuthors = from author in pubs.authors where author.state == "CA" select new { author.au_fname, author.au_lname }; foreach (var author in californianAuthors) { Console.WriteLine(author); } Despite appearances, C# doesn’t know anything about SQL or databases. To enable this syntax, C# 3.0 added a raft of language features which, in combination, allow code of this sort to be used not just for database access, but also for XML parsing, or working 4 | Chapter 1: Introducing C# with object models. Moreover, many of the individual features can be used in other contexts, as we’ll see in later chapters. C# prefers small, composable, general-purpose features over monolithic, specialized ones. A striking example of this philosophy is a feature that was demonstrated in prototype form in C#, but which eventually got left out: XML literals. This experimental syntax allowed inline XML, which compiled into code that built an object model representing that XML. The C# team’s decision to omit this feature illustrates a stylistic preference for generality over highly specialized features—while the LINQ syntax has many ap- plications, XML literal syntax cannot be used for anything other than XML, and this degree of specialization would feel out of place in C#.* Managed Code The .NET Framework provides more than just a class library. It also provides services in subtler ways that are not accessed explicitly through library calls. For example, earlier we mentioned that C# can automate some aspects of memory management, a notori- ous source of bugs in C++ code. Abandoning heap-allocated objects once you’re done with them is a coding error in C++, but it’s the normal way to free them in .NET. This service is provided by the CLR—the .NET Framework’s runtime environment. Al- though the C# compiler works closely with the runtime to make this possible, provid- ing the necessary information about how your code uses objects and data, it’s ultimately the runtime that does the work of garbage collection. Depending on what sorts of languages you may have worked with before, the idea that the language depends heavily on the runtime might seem either completely natural or somewhat disconcerting. It’s certainly different from how C and C++ work—with those languages, the compiler’s output can be executed directly by the computer, and although those languages have some runtime services, it’s possible to write code that can run without them. But C# code cannot even execute without the help of the run- time. Code that depends entirely on the runtime is called managed code. Managed compilers do not produce raw executable code. Instead, they produce an intermediate form of code called IL, the Intermediate Language.† The runtime decides exactly how to convert it into something executable. One practical upshot of managed code is that a compiled C# program can run on both 32-bit and 64-bit systems without modification, and can even run on different processor architectures—it’s often possible * VB.NET supports XML literals. Since C# 2.0 shipped, the C# and VB.NET teams have operated a policy of keeping the feature sets of the two languages similar, so the fact that VB.NET picked up a feature that C# abandoned shows a clear difference in language philosophy. † Depending on whether you read Microsoft’s documentation, or the ECMA CLI (Common Language Infrastructure) specifications that define the standardized parts of .NET and C#, IL’s proper name is either MSIL (Microsoft IL) or CIL (Common IL), respectively. The unofficial name, IL, seems more popular in practice. Language Style | 5 for code that runs on an ARM-based handheld device to run unmodified on Intel-based PCs, or on the PowerPC architecture found in the Xbox 360 game console. As interesting as CPU independence may be, in practice the most useful aspect of man- aged code and IL is that the .NET runtime can provide useful services that are very hard for traditional compilation systems to implement well. In other words, the point is to make developers more productive. The memory management mentioned earlier is just one example. Others include a security model that takes the origin of code into account rather than merely the identity of the user who happens to be running the code; flexible mechanisms for loading shared components with robust support for servicing and ver- sioning; runtime code optimization based on how the code is being used in practice rather than how the compiler guesses it might be used; and as already mentioned, the CLR’s ability to verify that code conforms to type safety rules before executing it, ruling out whole classes of security and stability bugs. If you’re a Java developer, all of this will sound rather familiar—just substitute byte- code for IL and the story is very similar. Indeed, a popular but somewhat ignorant “joke” among the less thoughtful members of the Java community is to describe C# as a poor imitation of Java. When the first version of C# appeared, the differences were subtle, but the fact that Java went on to copy several features from C# illustrates that C# was always more than a mere clone. The languages have grown more obviously different with each new version, but one difference, present from the start, is particularly im- portant for Windows developers: C# has always made it easy to get at the features of the underlying Windows platform. Continuity and the Windows Ecosystem Software development platforms do not succeed purely on their own merits—context matters. For example, widespread availability of third-party components and tools can make a platform significantly more compelling. Windows is perhaps the most striking example of this phenomenon. Any new programming system attempting to gain ac- ceptance has a considerable advantage if it can plug into some existing ecosystem, and one of the biggest differences between C# and Java is that C# and the .NET Framework positively embrace the Windows platform, while Java goes out of its way to insulate developers from the underlying OS. If you’re writing code to run on a specific operating system, it’s not especially helpful for a language to cut you off from the tools and components unique to your chosen platform. Rather than requiring developers to break with the past, .NET offers con- tinuity by making it possible to work directly with components and services either built into or built for Windows. Most of the time, you won’t need to use this—the class library provides wrappers for a lot of the underlying platform’s functionality. However, if you need to use a third-party component or a feature of the operating system that doesn’t yet have a .NET wrapper, the ability to work with such unmanaged features directly from managed code is invaluable. 6 | Chapter 1: Introducing C# While .NET offers features to ease integration with the underlying plat- form, there is still support for non-Windows systems. Microsoft’s Sil- verlight can run C# and VB.NET code on Mac OS X as well as Windows. There’s an open source project called Mono which enables .NET code to run on Linux, and the related Moonlight project is an open source version of Silverlight. So the presence of local platform integration fea- tures doesn’t stop C# from being useful on multiple platforms—if you want to target multiple operating systems, you would just choose not to use any platform-specific features. So the biggest philosophical difference between C# and Java is that C# provides equal support for direct use of operating-system-specific features and for platform independence. Java makes the former dispro- portionately harder than the latter. The latest version of C# contains features that enhance this capability further. Several of the new C# 4.0 features make it easier to interact with Office and other Windows applications that use COM automation—this was a weak spot in C# 3.0. The relative ease with which developers can reach outside the boundaries of managed code makes C# an attractive choice—it offers all the benefits of managed execution, but retains the ability to work with any code in the Windows environment, managed or not. C# 4.0, .NET 4, and Visual Studio 2010 Since C# favors general-purpose language features designed to be composed with one another, it often doesn’t make sense to describe individual new features on their own. So rather than devoting sections or whole chapters to new features, we cover them in context, integrated appropriately with other, older language features. The section you’re reading right now is an exception, of course, and the main reason is that we expect people already familiar with C# 3.0 to browse through this book in bookstores looking for our coverage of the new features. If that’s you, welcome to the book! If you look in the Preface you’ll find a guide to what’s where in the book, including a section just for you, describing where to find material about C# 4.0 features. That being said, a theme unites the new language features in version 4: they support dynamic programming, with a particular focus on making certain interoperability sce- narios simpler. For example, consider the C# 3.0 code in Example 1-3 that uses part of the Office object model to read the Author property from a Word document. Example 1-3. The horrors of Office interop before C# 4.0 static void Main(string[] args) { var wordApp = new Microsoft.Office.Interop.Word.Application(); object fileName = @"WordFile.docx"; object missing = System.Reflection.Missing.Value; C# 4.0, .NET 4, and Visual Studio 2010 | 7 object readOnly = true; Microsoft.Office.Interop.Word._Document doc = wordApp.Documents.Open(ref fileName, ref missing, ref readOnly, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing); object docProperties = doc.BuiltInDocumentProperties; Type docPropType = docProperties.GetType(); object authorProp = docPropType.InvokeMember("Item", BindingFlags.Default | BindingFlags.GetProperty, null, docProperties, new object[] { "Author" }); Type propType = authorProp.GetType(); string authorName = propType.InvokeMember("Value", BindingFlags.Default |BindingFlags.GetProperty, null, authorProp, new object[] { }).ToString(); object saveChanges = false; doc.Close(ref saveChanges, ref missing, ref missing); Console.WriteLine(authorName); } That’s some pretty horrible code—it’s hard to see what the example does because the goal is lost in the details. The reason it is so unpleasant is that Office’s programming model is designed for dynamic languages that can fill in a lot of the details at runtime. C# 3.0 wasn’t able to do this, so developers were forced to do all the work by hand. Example 1-4 shows how to do exactly the same job in C# 4.0. This is a lot easier to follow, because the code contains only the relevant details. It’s easy to see the sequence of operations—open the document, get its properties, retrieve the Author property’s value, and close the document. C# 4.0 is now able to fill in all the details for us, thanks to its new dynamic language features. Example 1-4. Office interop with C# 4.0 static void Main(string[] args) { var wordApp = new Microsoft.Office.Interop.Word.Application(); Microsoft.Office.Interop.Word._Document doc = wordApp.Documents.Open("WordFile.docx", ReadOnly: true); dynamic docProperties = doc.BuiltInDocumentProperties; string authorName = docProperties["Author"].Value; doc.Close(SaveChanges: false); Console.WriteLine(authorName); } 8 | Chapter 1: Introducing C# This example uses a couple of C# 4.0 features: it uses the new dynamic keyword for runtime binding to members. It also uses the support for optional arguments. The Open and Close methods take 16 and 3 arguments, respectively, and as you can see from Example 1-3, you need to provide all of them in C# 3.0. But Example 1-4 has only provided values for the arguments it wants to set to something other than the default. Besides using these two new features, a project containing this code would usually be built using a third new interop feature called no-PIA. There’s nothing to see in the preceding example, because when you enable no-PIA in a C# project, you do not need to modify your code—no-PIA is essentially a deployment feature. In C# 3.0, you had to install special support libraries called primary interop assemblies (PIAs) on the target machine to be able to use COM APIs such as Office automation, but in C# 4.0 you no longer have to do this. You still need these PIAs on your development machine, but the C# compiler can extract the information your code requires, and copy it into your application. This saves you from deploying PIAs to the target machine, hence the name, “no-PIA”. While these new language features are particularly well suited to COM automation interop scenarios, they can be used anywhere. (The “no-PIA” feature is narrower, but it’s really part of the .NET runtime rather than a C# language feature.) Summary In this chapter we provided a quick overview of the nature of the C# language, and we showed some of its strengths and how the latest version has evolved. There’s one last benefit you should be aware of before we get into the details in the next chapter, and that’s the sheer quantity of C# resources available on the Internet. When the .NET Framework first appeared, C# adoption took off much faster than the other .NET languages. Consequently, if you’re searching for examples of how to get things done, or solutions to problems, C# is an excellent choice because it’s so well represented in blogs, examples, tools, open source projects, and webcasts—Microsoft’s own docu- mentation is pretty evenhanded between C# and VB.NET, but on the Web as a whole, you’re far better served if you’re a C# developer. So with that in mind, we’ll now look at the fundamental elements of C# programs. Summary | 9 CHAPTER 2 Basic Programming Techniques To use a programming language, you must master the fundamentals. You need to un- derstand the elements required to construct a working program, and learn how to use the development tools to build and run code. You also need to become familiar with the everyday features for representing information, performing calculations, and mak- ing decisions. This chapter will introduce these core features of the C# language. Getting Started We’ll be working in Visual Studio, the Microsoft development environment. There are other ways to build C# programs, but Visual Studio is the most widely used and it’s freely available, so we’ll stick with that. If you don’t have Visual Studio, you can download the free Express edition from http://www.microsoft.com/express/. In the first part of this chapter, we’ll create a very simple program so that you can see the bare minimum of steps required to get up and running. We’ll also examine all of the pieces Visual Studio creates for you so that you know exactly what the development environment is doing for you. And then we’ll build some slightly more interesting ex- amples to explore the C# language. To create a new C# program, select the File→New Project menu option, or just use the Ctrl-Shift-N shortcut. This will open Visual Studio’s New Project dialog, shown in Figure 2-1, where you can pick the kind of program you want to build. In the Installed Templates list on the lefthand side, ensure that the Visual C# item is expanded, and inside that, select the Windows item—applications that run locally on Windows are the easiest to create. We’ll get into other kinds of programs such as web applications later in the book. 11 In the dialog’s center, select the Console Application template. This creates an old- fashioned command-line application that runs in a console window. It might not be the most exciting kind of program, but it’s the easiest to create and understand, so that’s where we’ll start. You need to pick a name for your program—by default, Visual Studio will suggest something unimaginative such as ConsoleApplication1. In the Name field near the bot- tom of the dialog, type HelloWorld. (OK, so that’s equally unimaginative, but at least it’s descriptive.) Visual Studio also wants to know where you’d like to put the project on your hard disk—put it wherever you like. It can also create a separate “solution” directory. That’s something you’d do in a larger program made up of multiple com- ponents, but for this simple example, you want the “Create directory for solution” checkbox to be unchecked. When you click the OK button, Visual Studio will create a new project, a collection of files that are used to build a program. C# projects always contain source code files, but they often include other types of files, such as bitmaps. This newly created project will contain a C# source file called Program.cs, which should be visible in Visual Studio’s text editor. In case you’re not following along in Visual Studio as you read this, the code is reproduced in Example 2-1. By the way, there’s no particular significance to the filename Program.cs. Visual Studio doesn’t care what you call your source files; by convention, they have a .cs extension, short for C#, although even that’s optional. Figure 2-1. Visual Studio’s New Project dialog 12 | Chapter 2: Basic Programming Techniques Example 2-1. The code in a freshly created console application using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace HelloWorld { class Program { static void Main(string[] args) { } } } This program doesn’t do anything yet. To turn it into the traditional first example, you’ll need to add one line of code. This will go in between the two lines that contain the most-indented pair of braces ({ and }). The modified version is shown in Exam- ple 2-2, with the new line in bold. Example 2-2. The traditional first example, “Hello, world” using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace HelloWorld { class Program { static void Main(string[] args) { Console.WriteLine("Hello, world"); } } } This example is now ready to run. From the Debug menu select the Start Without Debugging item, or just press Ctrl-F5. The program will run, and because you’ve writ- ten a console application, a console window will open. The first line of this window will contain the text “Hello, world” and this will be followed by a prompt saying “Press any key to continue...” Once you’ve finished admiring the fruits of your creation, press a key to dismiss the window. Don’t use Debug→Start Debugging or F5—this will run the application in Visual Studio’s debugging mode, which doesn’t keep the window open once the application has finished. That’s not helpful for this ex- ample, which will most likely run to completion and then close the window before you’ve had a chance to see the output. Getting Started | 13 Now that we have a complete program, let’s look at the code to see what each part is for—all of the pieces are things you’ll deal with every time you write in C#. Starting from the top, Program.cs has several lines beginning with using: using System; using System.Collections.Generic; using System.Linq; using System.Text; These using directives help the C# compiler work out what external code this particular source file will be using. No code is an island—to get any useful work done, your programs will rely on other code. All C# programs depend on the .NET Framework class library, for example: the one line of code we added to our program uses the class library to display a message. Using directives can declare an intent to use classes from any library—yours, Microsoft’s, or anyone’s. All the directives in our example start with System, which indicates that we want to use something from the .NET Framework. This text that follows the using keyword denotes a namespace. Namespaces and Types The .NET Framework class library is big. To make it easier to find your way around the many services it offers, the library is split into namespaces. For example, the System.IO namespace offers I/O (Input/Output) services such as working with files on disk, while System.Data.SqlClient is for connecting to a SQL Server database. A namespace contains types. A type typically represents either a kind of information or a kind of object. For example, there are types that provide the core forms of information used in all programs, such as System.String which represents text, or the various nu- meric types such as System.Double or System.Int32. Some types are more complex— for example, the System.Net.HttpWebRequest class represents an HTTP request to be sent to a web server. A few types do not represent any particular thing, but simply offer a set of services, such as the System.Math class, which provides mathematical functions such as Sin and Log, and constants such as π or the base of natural logarithms, e. (We will explore the nature of types, objects, and values in much more detail in the next chapter.) All types in the .NET Framework class library belong to a namespace. The purpose of a using directive is to save you from typing the namespace every single time you need to use a class. For example, in a file that has a using System; directive you can just write Math.PI to get the value of π, instead of using the full name, System.Math.PI. You’re not required to write using directives, by the way—if you happen to enjoy typing, you’re free to use the fully qualified name. But since some namespaces get quite long—for example, System.Windows.Media.Imaging—you can see how the shorthand enabled by a using directive can reduce clutter considerably. You might be wondering why namespaces are needed at all if the first thing we usually do is add a bunch of using directives to avoid having to mention the namespace 14 | Chapter 2: Basic Programming Techniques anywhere else. One reason is disambiguation—some type names crop up in multiple places. For example, the ASP.NET web framework has a type called Control, and so do both WPF and Windows Forms. They represent similar concepts, but they are used in completely different contexts (web applications versus Windows applications). Al- though all of these types are called Control, they are distinct thanks to being in different namespaces. This disambiguation also leaves you free to use whatever names you want in your own code even if some names happen to be used already in parts of the .NET class library you never knew existed. Since there are more than 10,000 types in the framework, it’s entirely possible that you might pick a name that’s already being used, but namespaces make this less of a problem. For example, there’s a Bold class in .NET, but if you happen not to be using part of the library it belongs to (WPF’s text services) you might well want to use the name Bold to mean something else in your own code. And since .NET’s own Bold type is hidden away in the System.Windows.Documents namespace, as long as you don’t add a using directive for that namespace you’re free to use the name Bold yourself to mean whatever you like. Even when there’s no ambiguity, namespaces help you find your way around the class library—related types tend to be grouped into one namespace, or a group of related namespaces. (For example, there are various namespaces starting with System.Web con- taining types used in ASP.NET web applications.) So rather than searching through thousands of types for what you need, you can browse through the namespaces—there are only a few hundred of those. You can see a complete list of .NET Framework class library namespa- ces, along with a short description of what each one is for, at http://msdn .microsoft.com/library/ms229335. Visual Studio adds four namespace directives to the Program.cs file in a new console project. The System namespace contains general-purpose services, including basic data types such as String, and various numeric types. It also contains the Console type our program uses to display its greeting and which provides other console-related services, such as reading keyboard input and choosing the color of your output text. The remaining three using directives aren’t used in our example. Visual Studio adds them to newly created projects because they are likely to be useful in many applications. The System.Collections.Generic namespace contains types for working with collec- tions of things, such as a list of numbers. The System.Linq namespace contains types used for LINQ, which provides convenient ways of processing collections of informa- tion in C#. And the System.Text namespace contains types useful for working with text. The using directives Visual Studio adds to a new C# file are there just to save you some typing. You are free to remove them if you happen not to be using those namespaces. And you can add more, of course. Namespaces and Types | 15 Removing Unwanted Using Directives There’s a quick way to remove unwanted using directives. If you right-click anywhere on your C# code, the context menu offers an Organize Usings item. This opens a submenu that includes a Remove Unused Usings item—this works out which using directives are surplus to requirements, and removes them. The submenu offers another option designed to appeal to those who like to keep their source code tidy—its Remove and Sort entry can remove unused using statements and then sort the rest into alpha- betical order. This menu is shown in Figure 2-2. Figure 2-2. Tidying up using directives The using directives are not the end of our simple program’s encounter with name- spaces. In fact, the very next line of code after these directives is also concerned with namespaces: namespace HelloWorld { ... } While using directives declare which namespaces our code consumes, this namespace keyword tells the compiler what namespace we plan to provide—the types we write in our programs belong to namespaces just like the types in the class library.* Here, Visual Studio has presumed that we’d like to put our code into a namespace named after the project we created. This is a common practice, although you’re free to use whatever * Strictly speaking, you can leave out the namespace, in which case your types will end up in the so-called global namespace. But this is considered a poor practice—you’ll normally want your own code to reap the same benefits that class libraries get from namespaces. 16 | Chapter 2: Basic Programming Techniques names you like for your namespaces—there’s no requirement that the namespace name match the program name. The C# compiler will even let you put your own code into namespaces whose names begin with System, but you should not do this (at least, not unless you work for Microsoft and are adding types to some future version of .NET’s class library). You’re likely to cause confusion if you break the convention that System namespaces contain .NET Framework types. Notice that the namespace is followed by an open brace ({). C# uses braces to denote containment—here, everything inside these braces will be in our HelloWorld name- space. Since namespaces contain types, it should come as no great surprise that the next line in the file defines a type. Specifically, it defines a class. The .NET Framework class library isn’t the only thing that gets to define classes—in fact, if you want to write any code at all in C# you must provide a type to contain that code. Some languages (such as C++) do not impose this constraint, but C# is an object- oriented (OO) programming language. We’ll explore OO concepts in the next chapter, but the main impact on our “Hello, world” example is that every bit of C# code must have a type that it calls home. There are a few different ways to define types in C#, which we’ll get to in the next few chapters, but for the present simple example, the distinctions are not yet relevant. So we use the most common, a class: class Program { ... } Again, note the braces—as with the namespace contents, the class’s contents are de- lineated by a pair of braces. We’re still not quite at the code yet—code lives inside a class, but more specifically, it must live inside a particular method inside a class. A method is a named block of code, which may optionally return some data. The class in our example defines a method called Main, and once again we use a pair of braces to show where it starts and ends: static void Main(string[] args) { ... } The first keyword here, static, tells C# that it’s not necessary to create a Program object (Program being the class that contains this method, remember) in order to use this method. As you’ll see in the next chapter, a lot of methods require an object, but our simple example doesn’t need one. Namespaces and Types | 17 The next keyword is void. This tells the compiler that our method doesn’t return any data—it just does some work. Many methods return information. For example, the System.Math class’s Cos method calculates the cosine of its input, and since it doesn’t know what you want to do with that result, it provides it as a return value—the output of the method. But the code in this example is rather more proactive than that—it decides to show a message on the screen, so there’s nothing for it to return.† On meth- ods that return data, you’d write the type of data being returned here, but since there’s nothing to return in this case, the nothingness is denoted by the void keyword. The next part, Main, is the name of the method. This happens to be a special name— the C# compiler will expect your program to provide one static method called Main, and it’ll run that method when the program is launched. The method name is followed by a parameter list, which declares the input the method requires. This particular example’s parameter list is (string[] args), which says that it expects just a single input and that the code will refer to it using the name args. It expects this input to be a sequence of text strings (the square brackets indicating that multiple strings may be passed instead of just one). As it happens, this particular pro- gram doesn’t use this input, but it’s a standard feature of the specially named Main method—command-line arguments are passed in here. We’ll return to this later in the chapter when we write a program that makes use of command-line arguments, but for now, our example doesn’t use it. So we’ll move on to the final part of the example— the code inside the Main method that was the one part we added to Visual Studio’s contributions and which represents the only work this program does: Console.WriteLine("Hello, world"); This shows the C# syntax for invoking a method. Here we’re using a method provided by the Console class, which is part of the .NET Framework class library, and it is defined in the System namespace. We could have written the fully qualified name, in which case the code would look like this: System.Console.WriteLine("Hello, world"); But because of the using System; directive earlier, we can use the shorter version—it means the same thing, it’s just more concise. The Console class provides the ability to display text in a console window and to read input typed by the user in an old-fashioned command-line application. In this case, we’re invoking the class’s WriteLine method, passing it the text "Hello, world". The WriteLine method will write whatever text we provide out to the console window. † This is the essential difference between the so-called functional and procedural approaches to coding, by the way. Code that just performs a computation or calculation and returns the result is called “functional” because it’s similar in nature to mathematical functions such as cosine, and square root. Procedural code tends to perform a sequence of actions. In some languages, such as F#, the functional style dominates, but C# programs typically use a mixture of both styles. 18 | Chapter 2: Basic Programming Techniques You’ll have noticed that the dot (.) is being used to mean different things here. We can use it to delineate the namespace name and the type name; for example, System.Console means the Console type in the System namespace. It can also be used to break up a namespace name, as in System.IO. Our example also uses it to indicate that we want to use a particular method provided by a class, as in Console.WriteLine. And as you’ll see, the dot turns up in a few other places in C#. Broadly speaking, the dot signifies that we want to use something that’s inside something else. The C# compiler works out from context exactly what that means. Although we picked over every line of code in this simple example, we haven’t quite finished exploring what Visual Studio did for us when we asked it to create a new application. To fully appreciate its work, we need to step out of the Program.cs source file and look at the whole project. Projects and Solutions It’s rare for a useful program to be so simple that you would want all of its source code in one file. You may occasionally stumble across horrors such as a single file containing tens of thousands of lines of code, but in the interest of quality (and sanity) it’s best to try to keep your source code in smaller, more manageable chunks—the larger and more complex anything gets the more likely it is to contain flaws. So Visual Studio is built to work with multiple source files, and it provides a couple of concepts for structuring your programs across those files: projects and solutions. A project is a collection of source files that the C# compiler combines to produce a single output—typically either an executable program or a library. (See the sidebar on the next page for more details on the compilation process.) The usual convention in Windows is that executable files have an .exe extension while libraries have a .dll ex- tension. (These extensions are short for executable and dynamic link library, respec- tively.) There isn’t a big difference between the two kinds of file; the main distinction is that an executable program is required to have an entry point—the Main function. A library is not something you’d run independently; it’s designed to be used by other programs, so a DLL doesn’t have its own entry point. Other than that, they’re pretty much the same thing—they’re just files that contain code and data. (The two types of file are so similar that you can use an executable as though it were a library.) So Visual Studio projects work in much the same way for programs and libraries. Projects and Solutions | 19 Source Code, Binary, and Compilation The .exe and .dll files produced by Visual Studio do not contain your source code. If you were to look at the HelloWorld.exe file produced by our example, it would not contain a copy of the text in the Program.cs file. C# is a compiled language, meaning that during the development process, the source is converted into a binary format that is easier for the computer to execute. Visual Studio compiled your code automatically when you ran the program earlier. Not all languages work this way. For example, JavaScript, a language used to add dy- namic behavior to web pages, does not need to be compiled—your web browser down- loads the source for any JavaScript required and runs it directly. But there are a few disadvantages with this. First, source code tends to be rather verbose—it’s important that source code be mean- ingful to humans as well as computers, because when we come to modify a program, we need to understand the code before changing it. But a computer can work with very dense binary representations of information, which makes it possible for compiled code to be much smaller than the source, thus taking up less space on disk and taking less time to download. Second, human-readable representations are relatively hard work for computers to process—computers are more at home with binary than with text. Compilation pro- vides the opportunity to convert all the human-readable text into a form more con- venient for the computer in advance. So compiled code tends to run faster than a system that works directly with the source. (In fact, although JavaScript was not designed to be compiled, modern JavaScript engines have taken to compiling script after down- loading it to speed things up. This still leaves it at a disadvantage to a language such as C# where compilation happens during development—when a script runs for the first time with such a system, the user of the web page has to wait while the script is down- loaded and compiled.) Some languages compile code into native machine language—the binary code that can be executed directly by a computer’s CPU. This offers a performance benefit: code compiled in this way doesn’t require any further processing to run. However, .NET languages don’t do this, because it limits where a compiled program can execute. As we mentioned in the first chapter, .NET languages compile into a so-called Intermediate Language (IL for short). This is a binary representation, so it’s compact and efficient for computers to process, but it’s not specific to any particular CPU type, enabling .NET programs to run on either 32-bit or 64-bit machines, or on different CPU architectures. The .NET Framework converts this IL into native machine language just before running it, a technique referred to as JIT (Just In Time) compilation. JIT compilation offers the best of both worlds: it’s much faster than compiling from the source, but it still retains the flexibility to target different machine types. 20 | Chapter 2: Basic Programming Techniques Some project types produce neither libraries nor executables. For ex- ample, there’s a project type for building .msi (Windows Installer) files from the outputs of other projects. So strictly speaking, a project is a fairly abstract idea: it takes some files and builds them into some kind of output. But projects containing C# code will produce either an EXE or a DLL. A solution is just a collection of related projects. If you are writing a library, you’ll probably want to write an application that uses it—even if the library is ultimately destined to be used by other people, you’ll still want to be able to try it out for testing and debugging purposes, so it’s useful to be able to have one or more applications that exercise the library’s functionality. By putting all of these projects into one solution, you can work with the DLL and its test applications all at once. By the way, Visual Studio always requires a solution—even if you’re building just one project, it is always contained in a solution. That’s why the project’s contents are shown in a panel called the Solution Explorer, shown in Figure 2-3. Figure 2-3. HelloWorld project in the Solution Explorer The Solution Explorer is usually visible on the righthand side of Visual Studio, but if you don’t see it you can open it with the View→Solution Explorer menu item. It shows all the projects in the solution—just the HelloWorld project in this example. And it shows all the files in the solution—you can see the Program.cs file we’ve been examining near the bottom of Figure 2-3. Farther up is an extra file we haven’t looked at, called AssemblyInfo.cs. If you open this you’ll see that Visual Studio puts version number and copyright information in that file—users will see this information if they view the com- piled output’s properties in Windows Explorer. Projects and Solutions | 21 You might find that on your system, the Solution Explorer doesn’t show the Solution node that’s visible at the top of Figure 2-3, and just shows the HelloWorld project. Visual Studio can be configured to hide the sol- ution when it contains just a single project. If you don’t see the solution and would like to, select the Tools→Options menu item, and in the Options dialog that opens select the Projects and Solutions item. One of the options will be the “Always show solution” checkbox—check this if you want to see the solution in the Solution Explorer even when you’ve got only one project. Besides the C# source files, the Solution Explorer as shown in Figure 2-3 also has a References section. This contains a list of all the libraries your project uses. By default, Visual Studio populates this with a list of DLLs from the .NET Framework class library that it thinks you might find useful. You might be experiencing déjà vu right now—didn’t we already tell the compiler which bits of the library we want with using directives? This is a common cause of confusion among developers learning C#. Namespaces are not libraries, and neither one is contained by the other. These facts are obscured by an apparent connection. For example, the System.Data library does in fact define a load of types in the System.Data namespace. But this is just a convention, and one that is only loosely followed. Libraries are often, but not always, named after the namespace with which they are most strongly associated, but it’s common for a library to define types in several different namespaces and it’s common for a namespace’s types to be distributed across several different li- braries. (If you’re wondering how this chaos emerged, see the sidebar below.) Namespaces and Libraries The distribution of types across DLLs in the class library is driven by a combination of efficiency requirements and history. The System.Core library is a good example of the latter. There is no System.Core namespace—this library defines types in numerous namespaces including System, System.IO, and System.Threading. But you’ll also find types in these same three namespaces in the System library and also a library called mscorlib. (All .NET programs have a reference to mscorlib, and since it’s mandatory, Visual Studio doesn’t show it in the Solution Explorer. It’s where critical types such as System.String and System.Int32 are defined.) One of the reasons System.Core exists as a separate DLL is that it first appeared in version 3.5 of .NET. With versions 3.0 and 3.5 of .NET, Microsoft chose to put completely new functionality into new DLLs rather than altering the DLLs that were provided in version 2.0. This packaging decision— choosing which types go in which DLLs—was independent from the conceptual deci- sion of which types belong in which namespaces. History doesn’t explain the whole story, though. Even the very first version of .NET split its namespaces across multiple libraries. One common reason for this was to avoid loading code that is never used. You wouldn’t want a desktop application to waste time and memory by loading the libraries for building web applications. In some cases, 22 | Chapter 2: Basic Programming Techniques namespaces are actually a pretty good guide to partitioning—chances are good that if you use one type from one of the System.Web namespaces, you’re going to be using lots of them. But there are a few cases in which namespaces are not the best way to determine packaging. For example, the System.Printing namespace is split across two libraries: the System.Printing library contains general print-related classes, but the ReachFrame work library adds extra types to the namespace that you may need if you’re working with a particular kind of printable document called an XPS file. If you’re not using that feature, you don’t need a reference to that specialized DLL. This raises a question: how do you know where to find things? It’s frustrating when adding a reference to the System.Printing library fails to give you access to the types in the System.Printing namespace that you were looking for. Fortunately, the help pages for each type tell you both the namespace and the library file (assembly) containing the type. The upshot is that the C# compiler cannot work out which libraries you want from your using directives, because in general it’s not possible to deduce which libraries are required from the namespaces alone. So a project needs to list which libraries it uses, and then individual source files in that project can declare which namespaces they are using. Visual Studio provides you with a set of references that it hopes will be useful, and for this very simple example, we’re not actually using most of them. Visual Studio notices when your code doesn’t use all of the libraries your project references, and automatically omits references to any unused libraries. This makes your binary slightly smaller than it would be if unnecessary references were left in. You can add or remove references to suit whatever program you’re building. To remove a reference, you can just select the library in the Solution Explorer and press the Delete key. (As it happens, our program is so simple that it depends only on the mandatory mscorlib library, so you could remove every DLL shown, and as long as you also remove any unused using directives from the source code, the program will still work.) To add a reference to a library, you can right-click on the References item and choose the Add Reference menu item. We’ll explore all of this in more detail in Chapter 15. It’s almost time to move on from “Hello, world” and start to explore more of the core language features, but first let’s recap what we’ve seen. The one line of executable code in our program invokes the WriteLine method of the System.Console class to print a message. This code lives inside a method whose special name, Main, marks it out as the method to run when the program starts. That method is contained by a class called Program, because C# requires all methods to belong to a type. This class is a member of the HelloWorld namespace, because we chose to follow the convention of having our namespace match the name of the compiled binary. Our program uses the using di- rectives supplied by Visual Studio to be able to refer to the Console class without needing to specify its namespace explicitly. So if you take one more look at the program, you Projects and Solutions | 23 now know what every single line is for. (It is reproduced in Example 2-3, with the unused using directives removed.) Example 2-3. “Hello, world” again (with fewer using directives) using System; namespace HelloWorld { class Program { static void Main(string[] args) { Console.WriteLine("Hello, world"); } } } With the whole example in one place, you can see clearly that the code is indented to reflect the structure. This is a common practice, but it’s not strictly necessary. As far as the C# compiler is concerned, when it comes to the space between elements of the language, there’s no difference between a single space, multiple spaces or tabs, or even blank lines—the syntax treats any contiguous quantity of whitespace as it would a single space.‡ So you are free to use space in your source code to improve legibility. This is why C# requires the use of braces to indicate containment, and it’s also why there’s a semicolon at the end of the line that prints out the message. Since C# doesn’t care whether we have one statement of code per line, split the code across multiple lines, or cram multiple statements onto one line, we need to be explicit about the end of each instruction, marking it with a ; so that the compiler knows where each new step of the program begins. Comments, Regions, and Readability While we’re looking at the structure and layout of source code, we need to examine a language feature that is extremely important, despite having precisely no effect on the behavior of your code. C# lets you add text to your source file that it will completely ignore. This might not sound important, or even useful, but it turns out to be vital if you want to have any hope of understanding code you wrote six months ago. There’s an unfortunate phenomenon known as “write-only code.” This is code that made some kind of sense to whoever wrote it at the time, but is incomprehensible to anyone trying to read it at a later date, even if the person reading it is its author. The best defense against this problem is to think carefully about the names you give the ‡ With the odd exception: in a string constant such as the “Hello, world” text in this example, whitespace is treated literally—C# presumes that if you put, say, three spaces in some text enclosed in double quotes, you really want three spaces. 24 | Chapter 2: Basic Programming Techniques features of your code and the way you structure your programs. You should strive to write your code so that it does what it looks like it does. Unfortunately, it’s sometimes necessary to do things in a nonobvious way, so even if your code is sufficiently clear that it’s easy to see what it does, it may not be at all clear why it does certain things. This tends to happen where your code meets other code— you might be interacting with a component or a service that’s idiosyncratic, or just plain buggy, and which works only if you do things in a particular way. For example, you might find that a component ignores the first attempt to do something and you need to add a redundant-looking line of code to get it to work: Frobnicator.SetTarget(""); Frobnicator.SetTarget("Norfolk"); The problem with this sort of thing is that it’s very hard for someone who comes across this code later on to know what to make of it. Is that apparently redundant line delib- erate? Is it safe to remove? Intrigue and ambiguity might make for engaging fiction, but these characteristics are rarely desirable in code. We need something to explain the mystery, and that’s the purpose of a comment. So you might write this: // Frobnicator v2.41 has a bug where it crashes occasionally if // we try to set the target to "Norfolk". Setting it to an empty // string first seems to work around the problem. Frobnicator.SetTarget(""); Frobnicator.SetTarget("Norfolk"); This is now less mysterious. Someone coming across this code knows why the appa- rently redundant line was added. It’s clear what problem it solves and the conditions under which that problem occurs, which makes it possible to find out whether the problem has been fixed in the most recent version of the offending component, making it possible to remove the fix. This makes it much easier to maintain code in the long run. As far as C# is concerned, this example is identical to the one without comments. The // character sequence tells it to ignore any further text up to the end of the line. So you can either put comments on their own line as shown earlier, or tack them onto the end of an existing line: Frobnicator.SetTarget(""); // Workaround for bug in v2.41 Like most of the C-family languages, C# supports two forms of comment syntax. As well as the single-line // form, you can write a comment that spans multiple lines, denoting the start with /* and the end with */, for example: /* This is part of a comment. This continues to be part of the same comment. Here endeth the comment. */ Comments, Regions, and Readability | 25 Bad Comments While comments can be very useful, many, sadly, are not. There are a couple of particularly common mistakes people make when writing comments, and it’s worth drawing attention to them so that you know what to avoid. Here’s the most common example: // Setting target to empty string Frobnicator.SetTarget(""); // Setting target to Norfolk Frobnicator.SetTarget("Norfolk"); These comments just repeat what the code already said. This is clearly a waste of space, but it’s surprisingly common, particularly from inexperienced developers. This may be because they’ve been told that comments are good, but they have no idea what makes a good comment. A comment should say something that’s not obvious from the code and which is likely to be useful to anyone trying to understand the code. The other common form of bad comment looks like this: // Setting target to Norfolk Frobnicator.SetTarget("Wiltshire"); Here, the comment contradicts the code. It seems like it shouldn’t be necessary to say that you shouldn’t do that, but it’s surprising how often you see this sort of thing in real code. It usually happens because someone modified the code without bothering to update the comment. A quick review of the comments after a code change is always worth doing. (Not least because if you’ve not paid enough attention to detail to notice that the comments are no longer accurate, chances are there are other problems you’ve not noticed.) XML Documentation Comments If you structure your comments in a certain way, Visual Studio is able to present the information in those comments in tool tips whenever developers use your code. As Example 2-4 shows, documentation comments are denoted with three slashes, and they contain XML elements describing the target of the comment—in this case, there’s a description of a method, its parameters, and the information it returns. Example 2-4. XML documentation comments ///

/// Returns the square of the specified number. ///

/// The number to square. /// The squared value. static double Square(double x) { return x * x; } 26 | Chapter 2: Basic Programming Techniques If a developer starts writing code to invoke this method, Visual Studio will show a pop up listing all available members matching what she’s typed so far, and also adds a tool tip showing the information from the

element of the selected method in the list, as Figure 2-4 shows. You’ll see similar information when using classes from the .NET Framework—documentation from its class libraries is provided as part of the .NET Framework SDK included with Visual Studio. (The C# compiler can extract this information from your source files and put it in a separate XML file, enabling you to provide the documentation for a library without necessarily having to ship the source code.) Figure 2-4. Summary information from XML documentation The information shows up as you start to type arguments, as Figure 2-5 shows. The information doesn’t appear here, but there are tools that can build doc- umentation from this information into HTML files or help files. For example, Microsoft provides a tool called Sandcastle, available from http://www.codeplex.com/Sandcastle, which can generate documentation with a similar structure to the documentation for Microsoft’s own class libraries. Figure 2-5. Parameter information from XML documentation We’re moving on from “Hello, world” now, so this is a good time to create a new project if you’re following along in Visual Studio as you read. (Select File→New Project or press Ctrl-Shift-N. Note that, by default, this will create a new solution for your new project. There’s an option in the New Project dialog to add the new project to the existing solution, but in this case, let it create a new one.) Create another console application and call it RaceInfo—the code is going to perform various jobs to analyze the perform- ance of a race car. Let Visual Studio create the project for you, and you’ll end up with much the same code as we had in Example 2-1, but with the Program class in a name- space called RaceInfo instead of HelloWorld. The first task will be to calculate the average speed and fuel consumption of the car, so we need to introduce the C# mechanism for holding and working with data. Comments, Regions, and Readability | 27 Variables C# methods can have named places to hold information. These are called variables, because the information they contain may be different each time the program runs, or your code may change a variable while the program runs. Example 2-5 defines three variables in our program’s Main method, to represent the distance traveled by the car, how long it has been moving, and how much fuel it has consumed so far. These variables don’t vary at all in this example—a variable’s value can change, but it’s OK to create variables whose value is fixed. Example 2-5. Variables static void Main(string[] args) { double kmTravelled = 5.14; double elapsedSeconds = 78.74; double fuelKilosConsumed = 2.7; } Notice that the variable names (kmTravelled, elapsedSeconds, and fuelKilosConsumed) are reasonably descriptive. In algebra it’s common to use single letters as variable names, but in code it is a good practice to use names that make it clear what the variable holds. If you can’t think of a good descriptive name for a variable, that’s often a symptom of trouble. It’s hard to write code that works if it’s not clear what information the code is working with. These names indicate not just what the variables represent, but also their units. This is of no significance to the compiler—we could call the three variables tom, dick, and harry for all it cares—but it’s useful for humans looking at the code. Misunderstandings about whether a particular value is in metric or imperial units have been known to cause some extremely expensive problems, such as the accidental destruction of spacecraft. This particular race team seems to use the metric system. (If you’re wondering why the fuel is in kilograms rather than, say, liters, it’s because in high-performance motor racing, fuel is typically measured by weight rather than volume, just like it is in aviation. Fuel tends to expand or contract as the temperature changes—you get better value for your money if you refill your car in the morning on a cold day than in the middle of a hot day—so mass is more useful because it’s a more stable measure.) Variable Types All three of the variable declarations in Example 2-5 start with the keyword double. This tells the compiler what kind of information the variable holds. For this example, we’re clearly working with numbers, but .NET offers several different numeric types. Table 2-1 shows the complete set, and it may look like a bewildering assortment of 28 | Chapter 2: Basic Programming Techniques options, but in practice the choice usually goes one of three ways: int, double, or decimal, which represent integers, floating-point, or decimal floating-point numbers, respectively. Table 2-1. Numeric types C# name .NET name Purpose float System.Single Whole numbers and a limited range of fractions, with a wide range of values thanks to “floating point.” Occupies 32 bits of space. double System.Double Double-precision version of float—same idea, but using 64 bits. byte System.Byte Non-negative integer. Occupies 8 bits. Represents values from 0 to 255. sbyte System.SByte Signed integer. Occupies 8 bits. Represents values from −128 to 127. short System.Int16 Signed integer. Occupies 16 bits. Represents values from −32,768 to 32,767. ushort System.UInt16 Non-negative integer. Occupies 16 bits. Represents values from 0 to 65,535. int System.Int32 Signed integer. Occupies 32 bits. Represents values from −2,147,483,648 to 2,147,483,647. uint System.UInt32 Nonnegative integer. Occupies 32 bits. Represents values from 0 to 4,294,967,295. long System.Int64 Signed integer. Occupies 64 bits. Represents values from −9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. ulong System.UInt64 Nonnegative integer. Occupies 64 bits. Represents values from 0 to 18,446,744,073,709,551,615. (none) System.Numer ics.BigInteger Signed integer. Grows in size as required. Value range limited only by available memory. decimal System.Decimal Supports whole numbers and fractions. Slightly less efficient than double, but provides more predictable behavior when using decimal fractions. Integers The int type (short for integer) represents whole numbers. That’s clearly no use for our example, because we’re dealing with numbers such as 5.14, and the closest that an int can get to that value is 5. But programs often deal with discrete quantities, such as the number of rows returned by a database query or the number of employees reporting to a particular manager. The principal advantage of an integer type is that it’s exact: there’s no scope for wondering if the number is really 5, or maybe just a number quite close to 5, such as 5.000001. Table 2-1 lists nine types capable of representing integers. The ninth, BigInteger, is a special case that we’ll get to later. The other eight support four different sizes, with a choice between the ability and inability to represent negative numbers. Unsigned numbers may seem less flexible, but they are potentially useful if you need to represent values that should never be negative. However, the unsigned integer types are not widely used—some programming languages don’t support them at all, and so you’ll find that the .NET Framework class library tends to use the signed types even Variables | 29 when the unsigned ones might make more sense. For example, the Count property available on most collection types is of type int—a signed 32-bit integer—even though it does not make sense for a collection to contain a negative number of items. Unsigned integers can also represent larger numbers than their signed equivalents. They don’t need to use up a bit to represent the sign, so they can use that to extend the range instead. However, this is something you should be wary of depending on. If you’re so close to the limits of a type’s range that one more bit makes a difference, you’re probably in danger of overflowing the type’s range in any case, and so you should consider a larger type. Besides the signed/unsigned distinction, the various types offer different sizes, and a correspondingly different range of values. 32 bits is a popular choice because it offers a usefully wide range of values and is efficient for a 32-bit processor to work with. 64- bit types are used for the (fairly rare) occasions when you’re dealing with large enough quantities that a 32-bit representation’s range of a couple of billion is insufficient. 16- bit values are rarely used, although they occasionally crop up when having to deal with old programming interfaces, file formats, or network protocols. The 8-bit byte type is important because binary I/O (e.g., working with files or network connections) is mostly byte-oriented. And for reasons of historical convention, bytes buck the trend in that the unsigned type is used more widely than the signed sbyte type. But outside of I/O, a byte is usually too small to be useful. So in practice, int is the most widely used integer type. The fact that C# even offers you all these other choices can seem a little archaic—it harks back to the time when computers had so little memory that 32-bit numbers looked like an expensive choice. It gets this from its C-family connections, but it does turn out to be useful to have this control when you need to work directly with Windows APIs, as you’ll see in Chapter 19. Notice that most of the types in Table 2-1 have two names. C# uses names such as int and long, but the .NET Framework calls these types by longer names such as System.Int32 and System.Int64. The shorter C# names are aliases, and C# is happy to let you use either. You can write this: int answer = 42; or this: System.Int32 answer = 42; or, if your C# source file has a using System; directive at the top, you can write this: Int32 answer = 42; 30 | Chapter 2: Basic Programming Techniques All of these are equivalent—they produce exactly the same compiled output. The last two are equivalent simply because of how namespaces work, but why does C# support a completely different set of aliases? The answer is historical: C# was designed to be easy to learn for people who are familiar with the so-called C family of languages, which includes C, C++, Java, and JavaScript. Most of the languages in this family use the same names for certain kinds of data types—most use the name int to denote a conveniently sized integer, for example. So C# is merely following suit—it allows you to write code that looks like it would in other C-family languages. By contrast, the .NET Framework supports many different languages, so it takes the prosaic approach of giving these numeric data types descriptive names—it calls a 32- bit integer System.Int32. Since C# lets you use either naming style, opinion is divided on the matter of which you should use.§ The C-family style (int, double, etc.) seems to be the more popular. Version 4 of the .NET Framework introduces an extra integer type that works slightly differently from the rest: BigInteger. It does not have a C-style name, so it’s known only by its class library name. Unlike all the other integer types, which occupy a fixed amount of memory that determines their range, a BigInteger can grow. As the number it represents gets larger, it simply consumes more space. The only theoretical limit on range is the amount of memory available, but in practice, the computational cost of working with vast numbers is likely to be the limiting factor. Even simple arithmetic operations such as multiplication can become rather expensive with sufficiently vast numbers. For example, if you have two numbers each with 1 million decimal digits— each number occupies more than 400 kilobytes of memory—multiplying these together takes more than a minute on a reasonably well-specified computer. BigInteger is useful for mathematical scenarios when you need to be able to work with very large numbers, but in more ordinary situations, int is the most popular integer type. Integers are all very well for countable quantities, but what if you need the ability to represent something other than a whole number? This is where floating-point types come in. Floating point The double and float types both offer the ability to support numbers with a fractional component. For example, you can represent the value 1.5 with either of these types, which you can’t do with any of the integer types. The only difference between double and float is the level of precision available: since floating-point numbers have a fixed size, they can offer only a limited amount of precision. This means that they cannot represent any fraction—the limited precision means floating-point numbers can only represent most numbers approximately. § Whenever more than one way of doing something exists in a programming system, a schism inevitably forms, offering the opportunity for long and pointless arguments over which is “better.” Variables | 31 Floating Point If you’re wondering why these are called floating-point types, the name is a technical description of how they work internally. These numbers contain a fixed number of binary digits to hold the value, and then another number that says where the . should go. So the point is a binary point, the binary equivalent of a decimal point. It’s float- ing because it can move around. A float offers about seven decimal places of precision, whereas a double offers about 17. (Strictly speaking, they offer 23 and 52 binary places of precision, respectively. These are binary formats, so their precision does not correspond to an exact number of decimal places of precision.) So the following code: double x = 1234.5678; double y = x + 0.0001; Console.WriteLine(x); Console.WriteLine(y); prints out what you’d expect: 1234.5678 1234.5679 If instead we use the float type: float x = 1234.5678f; float y = x + 0.0001f; Console.WriteLine(x); Console.WriteLine(y); we get this: 1234.568 1234.568 This often surprises new developers, but it’s normal, and is by no means unique to C#. If only a limited amount of space is available, you simply cannot represent all possible numbers with complete accuracy. Floating point, approximate as it is, is the standard way to represent noninteger numbers in most programming languages, and you’ll see this sort of inaccuracy anywhere. 32 | Chapter 2: Basic Programming Techniques Notice that when modifying the code to use float instead of double, we added the letter f to the end of the constants—0.0001f instead of just 0.0001, for example. This is because C# treats a number with a decimal point as a value of type double, and if we try to store this in a variable of type float, we risk losing data due to the lower precision. Such code is treated as an error, hence the need to explicitly tell C# that we know we’re working with single-precision floating-point values, with the f suffix. If you have a double you really would like to turn into a float, and you are prepared to tolerate the loss of precision, you can tell C# this with a cast operator. For example: double x = 1234.5678; double y = x + 0.0001; float impreciseSum = (float) (x + y); The (float) syntax here is a cast, an explicit instruction to the compiler that we want to convert the type. Since we are being explicit, the com- piler does not treat this as an error. For a lot of applications, limited precision is not too big a problem as long as you’re aware of it, but there’s a slightly subtler problem that afflicts double and float. They are both binary representations, because that’s the most efficient way of packing pre- cision into the space available. However, it means that you can get some surprising- looking results when working in decimal. For example, the number 0.1 cannot be rep- resented accurately as a finite-length binary fraction. (For much the same reason that 1/9 cannot accurately be represented as a finite-length decimal fraction. In either case, you end up with a recurring [i.e., infinitely long] number: 1/9 in decimal is 0.1111 recurring; 1/10 in decimal is 0.1, but in binary it’s 0.00011001100110011 recurring.) Take the following example: float f1 = 0.1f; float f2 = f1 + 0.1f; float f3 = f2 + 0.1f; float f4 = f3 + 0.1f; float f5 = f4 + 0.1f; float f6 = f5 + 0.1f; float f7 = f6 + 0.1f; float f8 = f7 + 0.1f; float f9 = f8 + 0.1f; Console.WriteLine(f1); Console.WriteLine(f2); Console.WriteLine(f3); Console.WriteLine(f4); Console.WriteLine(f5); Console.WriteLine(f6); Console.WriteLine(f7); Console.WriteLine(f8); Console.WriteLine(f9); Variables | 33 (We’ll see how to avoid such highly repetitive code when we get to loops later in the chapter, by the way.) This shows the following rather suspect output: 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8000001 0.9000001 The inability to represent 0.1 accurately is not initially obvious, because .NET rounds the numbers when displaying them, masking the problem. However, as we keep adding numbers together, the inaccuracies add up and eventually start to become visible. As you can imagine, accountants don’t like this sort of thing—if those numbers happened to represent fund transfers measured in billions of dollars, having $0.0000001 billion ($100) suddenly appear out of nowhere every eight transactions would be considered a bad practice. This is why there’s a special numeric type just for working in decimal. Decimal floating point The decimal type (or System.Decimal, as .NET calls it) is superficially very similar to double and float, except its internal representation is adapted to decimal representa- tions. It can represent up to 28 decimal digits of precision, and unlike the two binary floating-point types, any number that can be written as a 28-digit (or fewer) decimal can be represented completely accurately as a decimal variable. The value 0.1 fits com- fortably into 28 digits with room to spare, so this would fix the problem in the previous example. The decimal type still has limited precision; it just has less surprising behavior if you’re looking at all your numbers in decimal. So if you are performing calculations involving money, decimal is likely to be a better choice than double or float. The trade-off is that it’s slightly less efficient—computers are more at home in binary than decimal. For our race information application, we don’t have any particular need for decimal fidelity, which is why we’re using the double type in Example 2-5. Getting back to that example, recall that we defined three variables that hold the dis- tance our car has traveled, how long it took, and how much fuel it burned in the process. Here it is again so that you don’t have to flip back to it: static void Main(string[] args) { double kmTravelled = 5.141; double elapsedSeconds = 78.738; double fuelKilosConsumed = 2.7; } 34 | Chapter 2: Basic Programming Techniques Now that we’ve looked at the numeric types, the structure of these lines is pretty clear. We start with the type of data we’d like to work with, followed by the name we’d like to use, and then we use the = symbol to assign a value to the variable. But assigning constant values isn’t very exciting. You can get the computer to do more useful work, because you can assign an expression into a variable. Expressions and Statements An expression is a piece of code that produces a value of some kind. We’ve actually seen several examples already, the most basic being the numbers we’re assigning into the variables. So in our example, a number such as: 5.141 is an expression. Expressions where we just tell C# what value we want are called literal expressions. More interestingly, expressions can perform calculations. For ex- ample, we could calculate the distance traveled per kilogram of fuel consumed with the expression in Example 2-6. Example 2-6. Dividing one variable by another kmTravelled / fuelKilosConsumed The / symbol denotes division. Multiplication, addition, and subtraction are done with *, +, and -, respectively. You can combine expressions together too. The / operator requires two inputs—the dividend and the divisor—and each input is itself an expression. We were able to use variable names such as kmTravelled because a variable name is valid as an expression— the resultant value is just whatever that variable’s value is. But we could use literals, as Example 2-7 shows. (A trap awaits the unwary here; see the sidebar on the next page.) Example 2-7. Dividing one literal by another 60 / 10 Or we could use a mixture of literals and variable names to calculate the elapsed time in minutes: elapsedSeconds / 60 or a multiplication expression as one of the inputs to a division expression to calculate the elapsed time in hours: elapsedSeconds / (60 * 60) Expressions and Statements | 35 Integer Versus Floating-Point Division There’s a subtle difference between how division works in Examples 2-6 and 2-7. Since the two literals in Example 2-7 do not contain decimal points, the compiler treats them as integers, and so it will perform an integer division. But since the kmTravelled and fuelKilosConsumed variables are both floating-point, it will use a floating-point division operation. In this particular case it doesn’t matter, because dividing 60 by 10 produces another integer, 6. But what if the result had not been a whole number? If we had written this, for example: 3/4 the result would be 0, as this is an integer division—4 does not go into 3. However, given the following: double x = 3; double y = 4; the value of x/y would be 0.75, because C# would use floating-point division, which can deal with nonwhole results. If you wanted to use floating-point calculations with literals, you could write: 3.0/4.0 The decimal point indicates that we want floating-point numbers, and therefore float- ing-point division, so the result is 0.75. (The parentheses ensure that we divide by 60 * 60. Without the parentheses, this ex- pression would divide by 60, and then multiply by 60, which would be less useful. See the sidebar on the next page.) And then we could use this to work out the speed in kilometers per hour: kmTravelled / (elapsedSeconds / (60 * 60)) Expressions don’t actually do anything on their own. We have described a calculation, but the C# compiler needs to know what we want to do with the result. We can do various things with an expression. We could use it to initialize another variable: double kmPerHour = kmTravelled / (elapsedSeconds / (60 * 60)); or we could display the value of the expression in the console window: Console.WriteLine(kmTravelled / (elapsedSeconds / (60 * 60))); Both of these are examples of statements. Whereas an expression describes a calculation, a statement describes an action. In the last two examples, we used the same expression—a calculation of the race car’s speed— but the two statements did different things: one evaluated the expression and assigned it into a new variable, while the other evaluated the expression and then passed it to the Console class’s WriteLine method. 36 | Chapter 2: Basic Programming Techniques Order of Evaluation C# has a set of rules for working out the order in which to evaluate the components of an expression. It does not necessarily work from left to right, because some operators have a higher precedence than others. For example, imagine evaluating this: 1.0 + 3.0 / 4.0 from left to right. Start with 1.0, add 3.0 which gets you to 4.0, and then divide by 4.0— the result would be 1.0. But the conventional rules of arithmetic mean the result should be one and three quarters. And that’s just what C# produces—the result is 1.75. The division is performed before the addition, because division has higher precedence than division. Some groups of operators have equal precedence. For example, multiplication and division have equal precedence. When expressions contain multiple operations with the same precedence, mathematical operations are evaluated from left to right. So 10.0 / 2.0 * 5.0 evaluates to 25.0. But parentheses trump precedence, so 10.0 / (2.0 * 5.0) evaluates to 1.0. Some programming books go into great depths about all the details of precedence, but it makes for exceptionally tedious reading—C# has 15 different levels of precedence. The details are important for compiler writers, but of limited value for developers— code that relies heavily on precedence can be hard to read. Using parentheses to make evaluation order explicit can often improve clarity. But if you would like the gory details, you can find them at http://msdn.microsoft.com/en-us/library/aa691323. An expression’s type matters. The examples we just looked at involve numbers or numeric variables, and are of type double or int. Expressions can be of any type, though. For example, ("Hello, " + "world") is an expression of type string. If you wrote an assignment statement that tried to assign that expression into a variable of type double, the compiler would complain—it insists that expressions are either of the same type as the variable, or of a type that is implicitly convertible to the variable’s type. Implicit conversions exist for numeric types when the conversion won’t lose information—for example, a double can represent any value that an int can, so you’re allowed to assign an integer expression into a double variable. But attempting the opposite would cause a compiler error, because doubles can be larger than the highest int, and they can also contain fractional parts that would be lost. If you don’t mind the loss of information, you can put a cast in front of the expression: int approxKmPerHour = (int) kmPerHour; This casts the kmPerHour (which we declared earlier as a double) to an int, meaning it’ll force the value to fit in an integer, possibly losing information in the process. Expressions and Statements | 37 A variable doesn’t have to be stuck with its initial value for its whole life. We can assign new values at any time. Assignment Statements The previous section showed how to assign an expression’s value into a newly declared variable: double kmPerHour = kmTravelled / (elapsedSeconds / (60 * 60)); If at some later stage in the program’s execution new information becomes available, we could assign a new value into the kmPerHour variable—assignment statements aren’t required to declare new variables, and can assign into existing ones: kmPerHour = updateKmTravelled / (updatedElapsedSeconds / (60 * 60)); This overwrites the existing value in the kmPerHour variable. C# offers some specialized assignment statements that can make for slightly more suc- cinct code. For example, suppose you wanted to add the car’s latest lap time to the variable holding the total elapsed time. You could write this: elapsedSeconds = elapsedSeconds + latestLapTime; This evaluates the expression on the righthand side, and assigns the result to the vari- able specified on the lefthand side. However, this process of adding a value to a variable is so common that there’s a special syntax for it: elapsedSeconds += latestLapTime; This has exactly the same effect as the previous expression. There are equivalents for the other mathematical operators, so -= means to subtract the expression on the right from the variable on the left, *= does the same for multiplication, and so on. Increment and Decrement Operators While we’re looking at how to update values, we should also look at the increment and decrement operators. If we want to maintain a lap count, we could add one each time the car completes a lap: lapCount += 1; The C programming language’s designers considered adding one to be a sufficiently important case to devise an even more special syntax for it, called the increment oper- ator, which C# duly offers: lapCount++; There’s also a decrement operator, --, which subtracts one. This example is a state- ment, but you can also use the increment and decrement operators in the middle of an expression: int currentLap = lapCount++; 38 | Chapter 2: Basic Programming Techniques But be careful. The expression on the right of this assignment statement means “eval- uate the current value of lapCount and then increment lapCount after getting its current value.” So if lapCount was 3 before executing this statement, currentLap would be 3 and lapCount would be 4 after executing it. If you want to use the updated value, you put the increment (or decrement) operator before its target: int currentLap = ++lapCount; You could write a program that consisted entirely of variable declaration, assignment, increment, and method invocation statements. However, such a program wouldn’t be very interesting—it would always execute the same sequence of statements just once in the same order. Fortunately, C# provides some more interesting statements that allow a program to make decisions that dynamically change the flow of execution through the code. This is sometimes referred to as flow control. Flow Control with Selection Statements A selection statement selects which code path to execute next, based on the value of an expression. We could use a selection statement to work out whether the race car is likely to run out of fuel in the next few laps, and display a warning if it is. C# offers two selection statements: if statements and switch statements. To illustrate selection in action, we need to make a slight change to the program. Right now, our example hardcodes all of its data—the distance traveled, fuel consumed, and time elapsed are compiled into the code as literals. This makes selection statements uninteresting—the program would make the same decision every time because the data would always be the same. For the decision to be meaningful, we need to modify the program to accept input. Since we’re writing a console application, we can supply the necessary information as command-line arguments. We could run the program passing in the total distance, elapsed time, and fuel consumed, for example: RaceInfo 20.6 312.8 10.8 We can write a modified version of the program that picks up these command-line values instead of hardcoding them, as shown in Example 2-8. Example 2-8. Reading command-line inputs static void Main(string[] args) { double kmTravelled = double.Parse(args[0]); double elapsedSeconds = double.Parse(args[1]); double fuelKilosConsumed = double.Parse(args[2]); } There are a few interesting features to point out here before we add a selection state- ment. First, recall from earlier that the Main method, our program’s entry point, is passed a sequence of strings representing the command-line arguments in a variable called args. This sequence is an array, a .NET construct for holding multiple items of Flow Control with Selection Statements | 39 a particular type. (You can make arrays of anything—numbers, text, or any type. The string[] syntax indicates that this method expects an array of strings.) In an expression, we can retrieve a particular item from an array by specifying a number in square brackets after the array variable’s name. So the first three lines in our method here use args[0], args[1], and args[2] to get the first, second, and third items in the array—the three command-line arguments in this case. C-family languages tend to number things from zero, and C# follows suit. This may seem a little idiosyncratic, but it makes sense to the com- puter. You can think of it as saying how far into the array you want to look. If you want to look at the thing right at the start of the array, you don’t need to go any distance at all, so an offset of zero gets you the first item. If you’re British, you’ll recognize this logic from floor numbering—the first floor in a building in Great Britain is not the one at street level; you have to go up one flight of stairs to get to the first floor. Also notice the use of double.Parse. Command-line arguments are passed as text, be- cause the user can type anything: RaceInfo Jenson Button Rocks But our program expects numbers. We need to do something to convert the strings into numbers, and that’s what double.Parse does: it expects the text to contain a dec- imal number, and converts it into a double-precision floating-point representation of that number. (If you’re wondering what it would do if the text wasn’t in fact a number, it’ll throw an exception. Chapter 6 explains what that means and how to deal with it gracefully, but for now it means our program would crash with an error.) This example illustrates that method invocations can also be expressions—the double type’s Parse method returns a value of type double, meaning we can use it to initialize a variable of type double. But that’s all by the by—the point here is that our program now gets data that could be different each time the program runs. For example, a race engineer in the pit lane could run the program with new distance, timing, and fuel information each time the car completes a lap. So our program can now usefully make decisions based on its input using selection statements. One such statement is the if statement. if Statements An if statement is a selection statement that decides whether to execute a particular piece of code based on the value of an expression. We can use this to show a low-fuel warning by adding the code in Example 2-9 at the end of our example’s Main method. Most of the code performs calculations in preparation for making the decision. The if statement toward the end of the example makes the decision—it decides whether to execute the block of code enclosed in braces. 40 | Chapter 2: Basic Programming Techniques Example 2-9. if statement double fuelTankCapacityKilos = 80; double lapLength = 5.141; double fuelKilosPerKm = fuelKilosConsumed / kmTravelled; double fuelKilosRemaining = fuelTankCapacityKilos - fuelKilosConsumed; double predictedDistanceUntilOutOfFuel = fuelKilosRemaining / fuelKilosPerKm; double predictedLapsUntilOutOfFuel = predictedDistanceUntilOutOfFuel / lapLength; if (predictedLapsUntilOutOfFuel < 4) { Console.WriteLine("Low on fuel. Laps remaining: " + predictedLapsUntilOutOfFuel); } To test this, we need to run the program with command-line arguments. You could open a command prompt, move to the directory containing the built output of your project, and run it with the arguments you want. (It’ll be in the bin\Debug folder that Visual Studio creates inside your project’s folder.) Or you can get Visual Studio to pass arguments for you. To do that, go to the Solution Explorer panel and double-click on the Properties icon. This will open the project’s properties view, which has a series of tabs on the lefthand side. Select the Debug tab, and in the middle you’ll see a “Com- mand line arguments” text box as shown in Figure 2-6. Figure 2-6. Passing command-line arguments in Visual Studio If you run the program with arguments corresponding to just a few laps (e.g., 15 238 8) it won’t print anything. But try running it with the following arguments: 141.95 2156.2 75.6. It’ll predict that the car has about 1.6 laps of fuel remaining. The if state- ment in Example 2-9 tests the following expression: predictedLapsUntilOutOfFuel < 4 The < symbol means “less than.” So the code in braces following the if statement runs only if the number of predicted laps of fuel is less than 4. Clearly, 1.6 is less than 4, so in this case it’ll run that code, printing out the following: Low on fuel. Laps remaining: 1.60701035044548 Flow Control with Selection Statements | 41 You need to use the right kind of expression in an if statement. In this case, we’ve performed a comparison—we’re testing to see if a variable is less than 4. There are only two possible outcomes: either it’s less than 4 or it isn’t. So this expression is clearly different in nature to the expressions performing mathematical calculations. If you were to modify the program so that it prints the value of that expression: Console.WriteLine(predictedLapsUntilOutOfFuel < 4); it would display either True or False. The .NET Framework has a special type to rep- resent such an either/or choice, called System.Boolean, and as with the numeric types, C# defines its own alias for this type: bool.‖ An if statement requires a Boolean ex- pression. So if you try to use an expression with a numeric result, such as this: if (fuelTankCapacityKilos - fuelKilosConsumed) the compiler will complain with the error “Cannot implicitly convert type ‘double’ to ‘bool’.” This is its way of saying that it expects a bool—either true or false—and you’ve given it a number. In effect, that code says something like “If fourteen and a half then do this.” What would that even mean? The C language decided to answer that question by saying that 0 is equivalent to false, and anything else is equivalent to true. But that was only because it didn’t have a built-in Boolean type, so its if statement had to be able to work with numeric expressions. This turned out to be a frequent cause of bugs in C programs. Since C# does have a built-in bool type, it insists that an if statement’s expression is always of type bool. C# defines several operators which, like the < operator we used in Example 2-9, can compare two numbers to produce a Boolean true/false answer. Table 2-2 shows these. Some of these operators can be applied to non-numeric types too. For example, you can use the == and != operators to compare strings. (You might expect the other com- parison operators to work too, telling you whether one string would come before or after another when sorted alphabetically. However, there’s more than one way to sort strings—it turns out that the method used varies based on language and culture. And rather than have an expression such as text1 < text2 mean different things in different contexts, C# simply doesn’t allow it. If you want to compare strings, you have to call one of the methods provided by the String class that lets you say how you’d like the comparison to work.) ‖ The Boolean type is named after George Boole, who invented a branch of mathematical logic that uses just two values: true and false. His system is fundamental to the operation of all digital electronics, so it’s a shame that C# doesn’t see fit to spell his name properly. 42 | Chapter 2: Basic Programming Techniques Table 2-2. Comparison operators C# operator Meaning < Less than > Greater than <= Less than or equal to >= Greater than or equal to == Equal to != Not equal to Just as you can combine numeric expressions into more complex and powerful ex- pressions, C# provides operators that let you combine Boolean expressions to test multiple conditions. The && operator combines two Boolean expressions into a single expression that’s true only if both conditions are true. In our race example, we might use this to hide the low-fuel warning if we’re near the end of the race and the car has enough fuel to make it to the finish line. Imagine that we added an extra argument to pass in the number of remaining laps in the race, and an additional variable to hold that value; we could write: if ((predictedLapsUntilOutOfFuel < 4) && (predictedLapsUntilOutOfFuel < remainingLapsInRace)) { Console.WriteLine("Low on fuel. Laps remaining: " + predictedLapsUntilOutOfFuel); } This has the same effect as the following slightly more verbose code: if (predictedLapsUntilOutOfFuel < 4) { if (predictedLapsUntilOutOfFuel < remainingLapsInRace) { Console.WriteLine("Low on fuel. Laps remaining: " + predictedLapsUntilOutOfFuel); } } Only if both conditions are true will the message be displayed. There’s also a || oper- ator. Like &&, the || operator combines two Boolean expressions, but will be true if either of them is true. Flow Control with Selection Statements | 43 if...else The if statement examples we’ve looked at so far just decide whether to execute some optional code, but what if we want to choose between two actions? An if statement can optionally include an else section that runs if the condition was false, as in this hypothetical post-race example: if (weWonTheRace) { Sponsors.DemandMoreMoney(); } else { Driver.ReducePay(); } One type of if/else test comes up often enough that C-family languages have a special syntax for it: sometimes you want to pick between one of two values, based on some test. You could write this: string messageForDriver; if (weWonTheRace) { messageForDriver = "Congratulations"; } else { messageForDriver = "You're fired"; } Sometimes it’s more convenient to be able to put this inside an expression. This can be done with the ternary operator, so called because it contains three expressions: a Boo- lean test expression, the expression to use if the test is true, and the expression to use if the test is false. The syntax uses ? and : characters to separate the expressions, so the basic pattern is test ? resultIfTrue : resultIfFalse. We can collapse the previous if...else example to a single assignment statement by using the ternary operator in the expression on the righthand side of the assignment: string messageForDriver = weWonTheRace ? "Congratulations" : "You're fired"; You don’t have to space it out like this, by the way—we put the two options on separate lines to make them easy to see. But some people like to use the ternary operator to condense as much logic as possible into as little space as possible; this is either admir- able conciseness or impenetrable terseness, depending on your personal tastes. You can string multiple if...else tests together. To see how that might be useful in our example, consider how in motor racing, incidents or weather conditions may cause the race stewards to initiate certain safety procedures, such as temporarily disallowing overtaking maneuvers while wreckage is cleared from the track, releasing the safety car for the drivers to follow slowly if the wreckage is particularly spectacular, or in extreme 44 | Chapter 2: Basic Programming Techniques cases “red-flagging” the race—a temporary complete halt followed by a restart. Each of these has its own appropriate response, which can be dealt with by a chain of if...else if...else statements, as shown in Example 2-10. Example 2-10. Testing multiple conditions with if and else string raceStatus = args[3]; if (raceStatus == "YellowFlag") { Driver.TellNotToOvertake(); } else if (raceStatus == "SafetyCar") { Driver.WarnAboutSafetyCar(); } else if (raceStatus == "RedFlag") { if (ourDriverCausedIncident) { Factory.OrderNewCar(); Driver.ReducePay(); if (feelingGenerous) { Driver.Resuscitate(); } } else { Driver.CallBackToPit(); } } else { Driver.TellToDriveFaster(); } While this works, there’s an alternative. This pattern of choosing one option out of many is sufficiently common that C# has a special selection statement to handle it. switch and case Statements A switch statement lets you specify a list of expected values, and what to do for each value. The values can be either strings or integral types. (Integral types include int, short, etc.—you cannot switch on floating-point numbers. Enumeration types, which are discussed in Chapter 3, are considered to be integral types for the purposes of a switch statement.) We can use this to rewrite Example 2-10 as shown in Example 2-11. Flow Control with Selection Statements | 45 Example 2-11. Testing multiple conditions with switch and case string raceStatus = args[3]; switch (raceStatus) { case "YellowFlag": Driver.TellNotToOvertake(); break; case "SafetyCar": Driver.WarnAboutSafetyCar(); break; case "RedFlag": if (ourDriverCausedIncident) { Factory.OrderNewCar(); Driver.ReducePay(); if (feelingGenerous) { Driver.Resuscitate(); } } else { Driver.CallBackToPit(); } break; default: Driver.TellToDriveFaster(); break; } The break keyword you can see at the end of each case is present mainly for consistency with other C-like languages. In C and C++, if you leave off the break, the code will “fall” out of one case through to the next. So if we left off the break in the YellowFlag case, we’d end up telling drivers not to overtake and then warning them about the safety car. This would be a bug—and in general, you almost always don’t want fall- through. It’s unfortunate that in C and C++ fall-through was the default. C# changes this: if you want fall-through you must ask for it explicitly by writing goto case "SafetyCar". But despite fall-through no longer being the implicit default, you still need to write the same break state- ment as you would in other C-family languages when you don’t want fall-through—if you leave it out you’ll get an error. You might be wondering what is the point—this does exactly the same as Exam- ple 2-10, so why do we need a different syntax? As it happens, we don’t—there’s noth- ing you can do with switch and case that you can’t do with if and else. But switch and 46 | Chapter 2: Basic Programming Techniques case offer one useful advantage: they make it clear what we’re doing—we’re looking at a single expression (raceStatus) and we’re choosing one of a number of options based on the value of that expression. A developer familiar with C# can look at this code and understand the structure of the decision-making process at a glance. With the previous example, you would need to look at each else if statement in turn to make sure it wasn’t doing something more complex—chained else if statements are more flexible than switch statements, because each new link in the chain is allowed to test a com- pletely different expression, but that flexibility comes at the cost of making it harder to understand the code. Sometimes a self-imposed constraint can make code easier to read and maintain, and a switch statement is a good example of that. Selection statements make programs considerably more useful than they would oth- erwise be—they enable programs to make decisions. But our examples are still rather straightforward—they run just once, from start to finish, with the odd variation in the execution flow. The amount of work that is done is pretty trivial. So there’s another kind of statement that plays to a computer’s greatest strength: the ability to perform simple repetitive tasks many times over. Iteration Statements An iteration statement allows a sequence of other statements to be executed several times. (Repeated execution is also often known as a loop because, like the race car, the code goes round and round again.) This seems like it could be useful in our race data analysis—race cars usually complete many laps, so we will probably have multiple sets of data to process. It would be annoying to have to write the same code 60 times just to process all the data for a 60-lap race. Fortunately, we don’t have to—we can use one of C#’s iteration statements. Imagine that instead of passing in timing or fuel information as command-line argu- ments, the data was in files. We might have a text file containing one line per lap, with the elapsed time at the end of each lap. Another text file could contain the remaining fuel at the end of each lap. To illustrate how to work with such data, we’ll start with a simple example: finding the lap on which our driver went quickest. Since this code is a little different from the previous example, start a new project if you want to follow along. Make another console application called LapAnalysis. To be able to test our code we’ll need a file containing the timing information. You can add this to your Visual Studio project. Right-click on the LapAnalysis project in the Solution Explorer and select Add→New Item from the context menu. (Or just press Ctrl-Shift-A.) In the Installed Templates section on the left, select the General category under Visual C# Items, and then in the central area select Text File. Call the file LapTimes.txt and click Add. You’ll need this file to be somewhere the program can get to. Go to the Properties panel for the file—this is usually below the Solution Explorer panel, but if you don’t see it, right-click on LapTimes.txt in the Solution Explorer and Iteration Statements | 47 select Properties. In the Properties panel, you should see a Copy to Output Directory property. By default, this is set to “Do not copy”. Change it to “Copy if newer”—Visual Studio will ensure that an up-to-date copy of the file is available in the bin\Debug folder in which it builds your program. You’ll need some data in this file. We’ll be using the following—these numbers represent the elapsed time in seconds since the start of the race at the end of each lap: 78.73 157.2 237.1 313.8 390.7 470.2 The program is going to read in the contents of the file. To do this, it’ll need to use types from the System.IO namespace, so you’ll need to add the following near the top of your Program.cs file: using System.IO; Then inside the Main method, use the following code to read the contents of the file: string[] lines = File.ReadAllLines("LapTimes.txt"); The File type is in the System.IO namespace, and its ReadAllLines method reads in all the lines of a text file and returns an array of strings (string[]) with one entry per line. The easiest way to work through all these entries is with a foreach statement. foreach Statements A foreach statement executes a block of statements once for every item in a collection such as an array. For example, this: foreach (string line in lines) { Console.WriteLine(line); } will display every line of text from the lines array we just built. The block to execute each time around is, as ever, delimited by a { } pair. We have to provide the C# compiler with two things at the start of a foreach loop: the variable we’d like to use to access each item from the collection, and the collection itself. The string line part declares the first bit—the so-called iteration variable. And then the in lines part says that we want to iterate over the items in the lines array. So each time around the loop, line will contain the next string in lines. We can use this to discover the fastest lap time, as shown in Example 2-12. 48 | Chapter 2: Basic Programming Techniques Example 2-12. Finding the fastest lap with foreach string[] lines = File.ReadAllLines("LapTimes.txt"); double currentLapStartTime = 0; double fastestLapTime = 0; foreach (string line in lines) { double lapEndTime = double.Parse(line); double lapTime = lapEndTime - currentLapStartTime; if (fastestLapTime == 0 || lapTime < fastestLapTime) { fastestLapTime = lapTime; } currentLapStartTime = lapEndTime; } Console.WriteLine("Fastest lap time: " + fastestLapTime); The currentLapStartTime begins at zero, but is updated to the end time of the previous lap each time around the loop—we need this to work out how long each lap took, because each line of the file contains the total elapsed race time at each lap. And the fastestLapTime variable contains the time of the fastest lap yet found—it’ll be updated each time a faster lap is found. (We also update it when it’s zero, which it will be the first time we go around.) This finds the fastest lap time—76.7 seconds in the example data we’re using. But it doesn’t tell us which lap that was. Looking at the numbers, we can see that it happens to be the fourth, but it would be nice if the program could tell us. One way to do this is to declare a new variable called lapNumber, initializing it to 1 outside the loop, and adding one each time around, to keep track of the current lap. Then we can record the lap number on which we found the fastest time. Example 2-13 shows a modified ver- sion, with the additional code in bold. Example 2-13. Fastest lap including lap number string[] lines = File.ReadAllLines("LapTimes.txt"); double currentLapStartTime = 0; double fastestLapTime = 0; int lapNumber = 1; int fastestLapNumber = 0; foreach (string line in lines) { double lapEndTime = double.Parse(line); double lapTime = lapEndTime - currentLapStartTime; if (fastestLapTime == 0 || lapTime < fastestLapTime) { fastestLapTime = lapTime; fastestLapNumber = lapNumber; } currentLapStartTime = lapEndTime; lapNumber += 1; } Console.WriteLine("Fastest lap: " + fastestLapNumber); Console.WriteLine("Fastest lap time: " + fastestLapTime); Iteration Statements | 49 If you’re trying this out, this might be a good opportunity to acquaint yourself with Visual Studio’s debugging features—see the sidebar below. The Debugger When your code includes flow control statements that can vary the sequence of oper- ations, or how many times code runs, it can be useful to inspect the execution. If your code doesn’t work quite how you expect, you can watch what it does one line at a time by using Visual Studio’s built-in debugger. If instead of running the program normally you run it with the Debug→Step Into menu item (or the F11 keyboard shortcut if you’re using the C# profile for Visual Studio), it will run the code one line at a time—each time you choose Step Into, it will run one more line of code. And if you hover your mouse pointer over a variable, it will show you the current value, allowing you to see the current state of your program, as well as its current position. You can also arrange for the program to stop in the debugger when it reaches a particular point by setting a breakpoint, either by clicking in the left margin of the code editor or by putting the cursor on the line in question and selecting Debug→Toggle Breakpoint. A red dot appears in the margin to indicate that the code will stop when it reaches this point. Breakpoints are active only if you run the program from within the debugger, so you need to make sure you start with Debug→Start Debugging (or press F5) if you want breakpoints to work. Visual Studio’s debugger is a powerful and flexible system—these simple techniques barely scratch its surface, but they are very useful when trying to diagnose troublesome behavior in a program. Example 2-13 works well enough, but there’s an alternative iteration statement you can use for this sort of scenario: a for statement. for Statements A for statement is a loop in which some variable is initialized to a start value, and is modified each time around the loop. The loop will run for as long as some condition remains true—this means a for loop does not necessarily have to involve a collection, unlike a foreach loop. Example 2-14 is a simple loop that counts to 10. Example 2-14. Counting with a for loop for (int i = 1; i <= 10; i++) { Console.WriteLine(i); } Console.WriteLine("Coming, ready or not!"); The for keyword is followed by parentheses containing three pieces. First, a variable is declared and initialized. Then the condition is specified—this particular loop will 50 | Chapter 2: Basic Programming Techniques iterate for as long as the variable i is less than or equal to 10. You can use any Boolean expression here, just like in an if statement. And finally, there is a statement to be executed each time around the loop—adding one to i in this case. (As you saw earlier, i++ adds one to i. We could also have written i += 1, but the usual if arbitrary con- vention in C-style languages is to use the ++ operator here.) Earlier we recommended using variable names that are long enough to be descriptive, so you might be raising an eyebrow over the use of i as a variable name. There’s a convention with for loops where the iteration variable just counts up from zero—short variable names such as i, j, k, x, and y are often used. It’s not a universal convention, but you’ll see it widely used, particularly with short loops. We’re using this convention in Example 2-14 only because you will come across it sooner or later, and so we felt it was important to show it. But it’s arguably not an especially good way to write clear code, so feel free to choose more meaningful names in your own code. We could use this construct as an alternative way to find the fastest lap time, as shown in Example 2-15. Example 2-15. Finding the fastest lap with for string[] lines = File.ReadAllLines("LapTimes.txt"); double currentLapStartTime = 0; double fastestLapTime = 0; int fastestLapNumber = 0; for (int lapNumber = 1; lapNumber <= lines.Length; lapNumber++) { double lapEndTime = double.Parse(lines[lapNumber - 1]); double lapTime = lapEndTime - currentLapStartTime; if (fastestLapTime == 0 || lapTime < fastestLapTime) { fastestLapTime = lapTime; fastestLapNumber = lapNumber; } currentLapStartTime = lapEndTime; } Console.WriteLine("Fastest lap: " + fastestLapNumber); Console.WriteLine("Fastest lap time: " + fastestLapTime); This is pretty similar to the foreach example. It’s marginally shorter, but it’s also a little more awkward—our program is counting the laps starting from 1, but arrays in .NET start from zero, so the line that parses the value from the file has the slightly ungainly expression lines[lapNumber - 1] in it. (Incidentally, this example avoids using a short iteration variable name such as i because we’re numbering the laps from 1, not 0— short iteration variable names tend to be associated with zero-based counting.) Argu- ably, the foreach version was clearer, even if it was ever so slightly longer. The main Iteration Statements | 51 advantage of for is that it doesn’t require a collection, so it’s better suited to Exam- ple 2-14 than Example 2-15. while and do Statements C# offers a third kind of iteration statement: the while loop. This is like a simplified for loop—it has only the Boolean expression that decides whether to carry on looping, and does not have the variable initialization part, or the statement to execute each time around. (Or if you prefer, a for loop is a fancy version of a while loop—neither for nor foreach does anything you couldn’t achieve with a while loop and a little extra code.) Example 2-16 shows an alternative approach to working through the lines of a text file based on a while loop. Example 2-16. Iterating through a file with a while loop static void Main(string[] args) { using (StreamReader times = File.OpenText("LapTimes.txt")) { while (!times.EndOfStream) { string line = times.ReadLine(); double lapEndTime = double.Parse(line); Console.WriteLine(lapEndTime); } } } The while statement is well suited to the one-line-at-a-time approach. It doesn’t require a collection; it just loops until the condition becomes false. In this example, that means we loop until the StreamReader tells us we’ve reached the end of the file.# (Chap- ter 11 describes the use of types such as StreamReader in detail.) The exclamation mark (!) in front of the expression means not—you can put this in front of any Boolean expression to invert the result. So the loop runs for as long as we are not at the end of the stream. We could have used a for loop to implement this one-line-at-a-time loop—it also iterates until its condition becomes false. The while loop happens to be a better choice here simply because in this example, we have no use for the variable initialization or loop statement offered by for. #You’ll have noticed the using keyword on the line where we get hold of the StreamReader. We use this construct when it’s necessary to indicate exactly when we’ve finished with an object—in this case we need to say when we’re done with the file to avoid keeping operating system file handles open. 52 | Chapter 2: Basic Programming Techniques The approach in Example 2-16 would be better than the previous examples for a par- ticularly large file. The code can start working straight away without having to wait for the entire file to load, and it will use less memory because it doesn’t build the array containing every single line—it can hold just one line at a time in memory. For our example lap time file with just six lines of data, this won’t make any difference, but if you were processing a file with hundreds of thousands of entries, this while-based example could provide noticeably better performance than the array-based examples. This does not mean that while is faster than for or foreach. The per- formance difference here is a result of the code working with the file in a different way, and has nothing to do with the loop construct. In gen- eral, it’s a bad idea to focus on which language features are “fastest.” Performance usually depends on the way in which your code solves a problem, rather than which particular language feature you use. Note that for and while loops might never execute their contents at all. If the condition is false the first time around, they’ll skip the loop entirely. This is often desirable—if there’s no data, you probably want to do no work. But just occasionally it can be useful to write a loop that is guaranteed to execute at least once. We can do this with a variation on the while loop, called the do while loop: do { Console.WriteLine("Waiting..."); } while (DateTime.Now.Hour < 8); The while keyword and condition come at the end, and we mark the start of the loop with the do keyword. This loop always executes at least once, testing the condition at the end of each iteration instead of the start. So this code will repeatedly show the message “Waiting...” until the current time is 8:00 a.m. or later. If it’s already past 8:00 a.m., it’ll still write out “Waiting...” once. Breaking Out of a Loop It can sometimes be useful to abandon a loop earlier than its natural end. In the case of a foreach loop, this might mean stopping before you’ve processed every item in the collection. With for or while loops, you get to write the loop condition so that you can stop under whatever conditions you like, but it can sometimes be more convenient to put the code that makes a decision to abandon a loop somewhere inside the loop body rather than in the condition. For these eventualities, C# provides the break keyword. Iteration Statements | 53 We saw break already in a switch statement in Example 2-11—we used it to say that we’re done with the switch and want to break out of that statement. The break keyword does the same thing in a loop: using (StreamReader times = File.OpenText("LapTimes.txt")) { while (!times.EndOfStream) { string line = times.ReadLine(); if (line == "STOP!") { break; } double lapEndTime = double.Parse(line); Console.WriteLine(lapEndTime); } } This is the loop from Example 2-16, modified to stop if it comes across a line in the input file that contains the text “STOP!” This breaks out immediately, abandoning the rest of the loop and leaping straight to the first line of code after the enclosing loop’s closing brace. (In that case, this happens to be the enclosing using statement’s closing brace, which will close the file handle.) Some people regard this use of break as bad practice. It makes it harder to understand the loop. When a loop contains no break statements, you can understand its lifetime by looking at the while (or for, or foreach) part. But if there are break statements, you need to look at more of the code to get a complete understanding of when the loop will finish. More generally, flow control that jumps suddenly out of the middle of a construct is frowned upon, because it makes it much harder for some- one to understand how execution flows through a program, and pro- grams that are hard to understand tend to be buggy. The computer scientist Edsger Dijkstra submitted a short letter on this topic in 1968 to an academic journal, which was printed under a now infamous head- ing, “Go-to statement considered harmful”. If you’re interested in iconic pieces of computing history, or if you’d like a detailed explanation of exactly why this sort of jumpy flow control is problematic, you can find the original letter at http://www.cs.utexas.edu/users/EWD/ewd02xx/ EWD215.PDF. To recap what we’ve explored so far, we’ve seen how to work with variables to hold information, how to write expressions that perform calculations, how to use selection statements that decide what to do, and how to build iteration statements that can do things repeatedly. There’s one more basic C# programming feature we need to look at to cover the most important everyday coding features: methods. 54 | Chapter 2: Basic Programming Techniques Methods As we saw earlier, a method is a named block of code. We wrote a method already— the Main method that runs when our program starts. And we used methods provided by the .NET Framework class library, such as Console.WriteLine and File.ReadAll Lines. But we haven’t looked at how and why you would introduce new methods other than Main into your own code. Methods are an essential mechanism for reducing your code’s complexity and enhanc- ing its readability. By putting a section of code into its own method with a carefully chosen name that describes what the method does, you can make it much easier for someone looking at the code to work out what your program is meant to do. Also, methods can help avoid repetition—if you need to do similar work in multiple places, a method can help you reuse code. In our race car example, there’s a job we may need to do multiple times: reading in numeric values from a file. We did this for timing information, but we’re going to need to do the same with fuel consumption and distance. Rather than writing three almost identical bits of code, we can put the majority of the code into a single method. The first thing we need to do is declare the method—we need to pick a name, define the information that comes into the method, and optionally define the information that comes back out. Let’s call the method ReadNumbersFromFile, since that’s what it’s going to do. Its input will be a text string containing the filename, and it will return an array of double-precision floating-point numbers. The method declaration, which will go inside our Program class, will look like this: static double[] ReadNumbersFromFile(string fileName) As you may recall from the discussion of Main earlier, the static keyword indicates that we do not need an instance of the containing Program type to be created for this method to run. (We’ll be looking at nonstatic methods in the next chapter when we start dealing with objects.) C# follows the C-family convention that the kind of data coming out of the method is specified before the name and the inputs, so next we have double[], indicating that this method returns an array of numbers. Then we have the name, and then in parentheses, the inputs required by this method. In this example there’s just one, the filename, but this would be a comma-separated list if more inputs were required. After the method declaration comes the method body—the statements that make up the method, enclosed in braces. The code isn’t going to be quite the same as what we’ve seen so far—up until now, we’ve converted the text to numbers one at a time imme- diately before processing them. But this code is going to return an array of numbers, just like File.ReadAllLines returns an array of strings. So our code needs to build up that array. Example 2-17 shows one way of doing this. Methods | 55 Example 2-17. A method for reading numbers from a file static double[] ReadNumbersFromFile(string fileName) { List numbers = new List(); using (StreamReader file = File.OpenText(fileName)) { while (!file.EndOfStream) { string line = file.ReadLine(); // Skip blank lines if (!string.IsNullOrEmpty(line)) { numbers.Add(double.Parse(line)); } } } return numbers.ToArray(); } This looks pretty similar to the example while loop we saw earlier, with one addition: we’re creating an object that lets us build up a collection of numbers one at a time— a List. It’s similar to an array (a double[]), but an array needs you to know how many items you want up front—you can’t add more items onto an existing array. The advantage of a List is that you can just keep adding new numbers at will. That matters here because if you look closely you’ll see we’ve modified the code to skip over blank lines, which means that we actually don’t know how many numbers we’re going to get until we’ve read the whole file. Once you’re done adding numbers to a list, you can call its ToArray() method to get an array of the correct size. This list class is an example of a collection class. .NET offers several of these, and they are so extremely useful that Chapters 7, 8, and 9 are related to working with collections. Notice the return keyword near the end of Example 2-17. This is how we return the information calculated by our method to whatever code calls the method. As well as specifying the value to return, the return keyword causes the current method to exit immediately, and for execution to continue back in the calling method. (In methods with a void return type, which do not return any value, you can use the return keyword without an argument to exit the method. Or you can just let execution run to the end of the method, and it will return implicitly.) If you’re wondering how the method re- members where it’s supposed to go back to, see the sidebar on the next page. With the ReadNumbersFromFile method in place, we can now write this sort of code: double[] lapTimes = ReadNumbersFromFile("LapTimes.txt"); double[] fuelLevels = ReadNumbersFromFile("FuelRemainingByLap.txt"); 56 | Chapter 2: Basic Programming Techniques The Call Stack When you invoke a method, the CLR allocates some memory to keep track of that method’s state. This state includes incoming arguments and local variables. When a method calls out to another method, the method state also remembers where we were in the calling method’s code to be able to carry on later. If you have nested method calls—if a first method calls a second method which calls a third method, for example—you end up with a sequence of method states, and this sequence is often referred to as the call stack. In general, a stack is a sequence of items where you can add or remove items only at the end of the sequence; by convention, we use the terms push and pop to describe adding and removing stack items. So when C# code invokes a new method, it pushes a new method state record onto the call stack. When the method returns, either because execution reaches the end or because we’ve hit a return statement, the current method state is popped from the call stack, and then execution resumes from where the previous method state record says the calling method had reached. You can look at the call stack in the Visual Studio debugger. The Debug→Win- dows→Call Stack menu item displays a window showing a list of all the current methods in the call stack. You can double-click on any of these items to see the current location, and if you’ve opened any of the debug windows that show local variable state from the Debug→Windows menu, these will show local variables for the method you select. It doesn’t take a lot of effort to understand that this code is reading in numbers for lap times and fuel levels from a couple of text files—the code makes this aspect of its behavior much clearer than, say, Example 2-12. When code does what it says it does, you make life much easier for anyone who has to look at the code after you’ve written it. And since that probably includes you, you’ll make your life easier in the long run by moving functionality into carefully named methods. This idea of moving code out of the middle of one method and into a separate method is very common, and is an example of refactoring. Generally speaking, refactoring means restructuring code without changing its behavior, to either simplify it, make it easier to understand and maintain, or avoid duplication. There are so many ways to refactor code that whole books have been written on the topic, but this particular refactoring operation is so useful that Visual Studio can automate it. If you select some code and then right-click on the C# editor window, it offers a Refactor→Extract Method menu item that does this for you. In practice, it’s not always that straightforward—you might need to re- structure the code a little first, before you’re in a position to factor out the pieces you’d like to move into a method. Example 2-17 had to work slightly differently from any of the previous examples to package the code into a reusable method. But while it may require some work, it’s a useful technique to apply. Methods | 57 Summary In this chapter, we looked at some of the most important concepts involved in the everyday writing of C#. We saw how to create and run projects in Visual Studio. We saw how namespaces help us work with the .NET Framework class library and other external code, without getting lost in the thousands of classes on offer. We used vari- ables and expressions to store and perform calculations with data. We used selection statements to make decisions based on input, and iteration statements to perform re- petitive work on collections of data. And we saw how splitting your code into well- named methods can enhance the reusability and readability of your code. In the next chapter, we’ll step outside the world of methods and look at C#’s support for object- oriented programming. 58 | Chapter 2: Basic Programming Techniques CHAPTER 3 Abstracting Ideas with Classes and Structs In the previous couple of chapters, we looked at some basic programming techniques such as loops and conditions, and used some of the data types built into the language and platform, such as int and string. Unfortunately, real programs—even fairly simple ones—are much, much more com- plicated than the examples we’ve built so far. They need to model the behavior of real- world objects like cars and planes, or ideas like mathematical expressions, or behaviors, like the transaction between you and your favorite coffee shop when you buy a double espresso and a brownie with your bank card. Divide and Conquer The best way to manage this complexity is to break a system down into manageable pieces, where each piece is small enough for us to understand completely. We should aim to craft each piece so that it fits neatly into the system as a whole with a small enough number of connections to the other pieces that we can comprehend all of those too. Abstracting Ideas with Methods We’ve already seen one tool for dividing our code into manageable pieces: methods. A method is a piece of a program that encapsulates a particular behavior completely. It’s worth understanding the benefits of methods, because the same principles apply to the classes and structs that are this chapter’s main subject. 59 You will often see the term function used instead of method; they’re related, but not identical. A function is a method that returns something. Some methods just do some work, and do not return any value. So in C#, all functions are methods, but not all methods are functions. Methods offer a contract: if we meet particular conditions, a method will do certain things for us. Conditions come in various forms: we might need to pass arguments of suitable types, perhaps with limits on the range (e.g., negative numbers may not be allowed). We may need to ensure certain things about the program’s environment— maybe we need to check that certain directories exist on disk, or that there’s sufficient free memory and disk space. There may be constraints on when we are allowed to call the method—perhaps we’re not allowed to call it if some related work we started earlier hasn’t completed yet. Likewise, there are several ways in which a method can hold up its side of the bargain. Perhaps it will just return a string or a number that is the result of a calculation involving the method’s inputs. It might change the state of some entity in our system in some way, such as modifying an employee’s salary. It may change something about the sys- tem environment—the method might install a new device driver, or change the current user’s color scheme, for example. Some methods interact with the outside world by sending messages over the network. Some aspects of the contract are formalized—a method’s parameter list defines the number and type of arguments we need to pass, for example, and its return type tells us what, if anything, to expect as a return value. But most of the contract is informally specified—we rely on documentation (or sometimes, conversations with the developer who wrote the method) to understand the full contract. But understand it we must, because the contract is at the heart of how methods make our lives easier. Methods simplify things for us in two ways. If we are the user of a method, then, as long as its internal implementation conforms to the contract, we can treat it as a “black box.” We call it, we expect it to work as described, and we don’t need to worry about how it worked. All its internal complexity is hidden from us, freeing us to think about ideas like “increase this employee’s salary,” without getting bogged down by details such as “open a connection to the database and execute some SQL.” If, on the other hand, we are the developer of a method, we don’t need to worry about who might call us, and why. As long as our implementation works as promised, we can choose any means of implementation we like—perhaps optimizing for speed, or size, or (more often than not) simplicity and maintainability. We can concentrate on details like whether we’re using the right connection string, and whether the SQL query modi- fies the database as intended, without needing to ask ourselves questions like “should we even be adjusting this particular employee’s salary at all?” So, one objective of good design is to hide distracting details and expose a simple model to your client. This practice is called encapsulation, and it’s harder than it looks. 60 | Chapter 3: Abstracting Ideas with Classes and Structs As is so often the case in life, making something look easy takes years of practice and hard work. It can also be a thankless task: if you devise a contract that is a model of clarity, people will probably think it was easy to design. Conversely, unnecessary com- plexity is often mistaken for cleverness. While methods are essential for achieving encapsulation, they do not guarantee it. It’s all too easy to write methods whose contract is unclear. This often happens when developers do something as an afterthought— it can be oh so tempting to add a bit of extra code to an existing method as a quick solution to a problem, but this risks making that’s method’s responsibilities less clear. A method’s name is often a good indicator of the clarity of the contract— if the name is vague, or worse, if it’s an inaccurate description of what the method does, you’re probably looking at a method that does a bad job of encapsulation. One of the great things about methods is that we can use them to keep breaking things into smaller and smaller pieces. Suppose we have some method called PlaceOrder, which has a well-defined responsibility, but which is getting a bit complicated. We can just split its implementation into smaller methods—say, CheckCustomerCredit, AllocateStock, and IssueRequestToWarehouse. These smaller methods do different bits of the work for us. This general technique, sometimes called functional decomposition, has a long history in mathematics. It was explored academically in computing applications as early as the 1930s. Bearing in mind that the first working programmable computers didn’t appear until the 1940s, that’s quite a pedigree. In fact, it has been around for so long that it now seems “obvious” to most people who have had anything to do with computer programming. That’s not the end of the story, though. Methods are great for describing the dynam- ics of a system—how things change in response to particular input data (the method arguments), and the results of those changes (a function’s return value, or a method’s side effects). What they’re not so good at is describing the current state of the system. If we examine a set of functions and a load of variables, how can we work out which pieces of information are supposed to be operated on by which functions? If methods were the only tool available for abstraction, we’d have a hard time telling the difference between the double that describes my blood pressure, and can be operated on by this method: void LowerMyBloodPressure(double pressureDelta) and the double that describes my weight and can be affected by this method: void EatSomeDonuts(int quantityOfDonuts) As programs get ever larger, the number of system state variables floating around in- creases, and the number of methods can explode exponentially. But the problems aren’t Divide and Conquer | 61 just about the sheer number of functions and variables you end up with. As you try to model a more complex system, it becomes harder to work out which functions and variables you actually need—what is a good “decomposition” of the system? Which methods relate to one another, and to which variables? Abstracting Ideas with Objects and Classes In the 1960s, two guys called Dahl and Nygaard (they’re Norwegian) were working on big simulation systems and were struggling with this problem. Because they worked on simulating real things, they realized that their code would be easier to understand if they had some clear way to group together all of the data and functions related to a particular type of real thing (or a particular object, we might say). They designed a programming language that could do this, called Simula 67 (after the year of its birth), and it is generally recognized as the grandmother of all the languages we’d call object-oriented, which (of course) includes C#. They had hit upon two important concepts: • The class: a description of a collection of data and the functions that operate on them • The object: an instance of a collection of data and the functions that operate on them (i.e., an instance of a class) With these simple ideas, we can remove all doubt over which functions operate on which data—the class describes for us exactly what goes with what, and we can handle multiple entities of the same kind by creating several objects of a particular class. Object-oriented analysis As an example, let’s think about a very simple computer system that maintains the information for an air traffic control (ATC) operation. (Safety notice: if you happen to be building an ATC system, I strongly recommend that you don’t base it on this one.) How does (this particular, slightly peculiar) ATC system work? It turns out that we’ve got a bunch of people in a big room in Washington, tracking a large number of planes that buzz around the airport in Seattle. Each plane has an identifier (BA0049, which flies in from London Heathrow, for instance). We need to know the plane’s position, which we’ll represent using three numbers: an altitude (in feet); the distance from the airport control tower (in miles); and a compass heading (measured in degrees from North), which will also be relative to the tower. Just to be clear, that’s not the direction the aircraft itself is facing—it’s the direction we’d have to face in order to be looking at the plane if we’re standing in the tower. We also need to know whether the aircraft is coming in to us, or away from us, and how fast. This, apparently, is quite important. (A more comprehensive model might include a second compass heading, representing 62 | Chapter 3: Abstracting Ideas with Classes and Structs the exact direction the plane is facing. But to keep this example simple, we’ll just track whether planes are approaching or departing.) As the planes come in, the controllers give them permission to take off or land, and instruct them to change their heading, height, or speed. The aim is to avoid them hitting each other at any point. This, apparently, is also quite important. At present they have a system where each controller is responsible for a particular piece of airspace. They have a rack which contains little slips of plastic with the aircraft’s ID on it, ordered by the height at which they are flying. If they are coming in to the airport, they use a piece of blue plastic. If they are going away, they use white plastic. To keep track of the heading, distance, and speed, they just write on the slip with a china graph pencil.* If the plane moves out of their airspace, they hand the plane over to another controller, who slips it into his own rack. So that’s our specification. In reality, a safety-critical system such as ATC would have a more robust spec. However, when lives are not at stake, software specifications are often pretty nebulous, so this example is, sadly, a fair representation of what to expect on your average software project. Armed with this brilliant description we need to come up with a design for a program which can model the system. We’re going to do that using object-oriented techniques. When we do an object-oriented analysis we’re looking for the different classes of object that we are going to describe. Very often, they will correspond to real things in the system. For a class to represent these real objects properly, we need to work out what information it is going to hold, and what functions it will define to manipulate that information. In general, any one piece of information will belong to exactly one object, of exactly one class. Not all of your classes will represent real-world objects. Some will relate to more abstract concepts like collections, or commands. However, de- signs that wander too far into the realms of the wholly abstract are often “clever” but not necessarily “good”. In our ATC example, it’s clear that we have a whole lot of different planes buzzing round the airport. It would therefore seem logical that we would model each one as an object, for which we would define a class called Plane. Because C# is a language with object-oriented features, we have a simple and expressive way of doing that. * A special kind of crayon, designed for writing on glossy surfaces such as plastic. Divide and Conquer | 63 Defining Classes We can start out with the simplest possible class. It will have no methods, and no data, so as a model of a plane in our system, it leaves something to be desired, but it gets us started. If you want to build your own version as you read, create a new Console Application project just as we did in Chapter 2. To add a new class, use the Project→Add Class menu item (or right-click on the project in the Solution Explorer and select Add→Class). It’ll add a new file for the class, and if we call it Plane.cs, Visual Studio will create a new source file with the usual using directives and namespace declaration. And most im- portantly, the file will contain a new, empty class definition, as shown in Example 3-1. Example 3-1. The empty Plane class class Plane { } Right; if we look back at the specification, there’s clearly a whole bunch of information we’ve got about the plane that we need to store somewhere. C# gives us a handy mechanism for this called a property. Representing State with Properties Each plane has an identifier which is just a string of letters and numbers. We’ve already seen a built-in type ideal for representing this kind of data: string. So, we can add a property called Identifier, of type string, as Example 3-2 shows. Example 3-2. Adding a property class Plane { string Identifier { get; set; } } A property definition always states the type of data the property holds (string in this case), followed by its name. By convention, we use PascalCasing for this name—see the sidebar on the next page. As with most nontrivial elements of a C# program, this is followed by a pair of braces, and inside these we say that we want to provide a get- ter and a set-ter for the property. You might be wondering why we need to declare these—wouldn’t any property need to be gettable and settable? But as we’ll see, these explicit declarations turn out to be useful. 64 | Chapter 3: Abstracting Ideas with Classes and Structs PascalCasing and camelCasing Most programming languages, including C#, use whitespace to separate elements of the code—it must be clear where one statement (or keyword, variable, or whatever) ends and the next begins, and we often rely on spaces to mark the boundaries. But this gives us a problem when it comes to naming. Lots of features of a program have names—classes, methods, properties, and variables, for example—and we might want to use multiple words in a name. But we can’t put a space in the middle of a name like this: class Jumbo Jet { } The C# compiler would complain—the space after Jumbo marks the end of the name, and the compiler doesn’t understand why we’ve put a second name, Jet, after that. If we want to use multiple words in a name, we have to do it without using spaces. C# programmers conventionally use two styles of capitalization to put multiple words in a name: • PascalCasing, where each word starts with a capital letter. This is used for types, properties, and methods. • camelCasing, where the first word starts with a lowercase letter and all subsequent words get a capital. This is used for parameters and fields. Pascal casing takes its name from the fact that it was a popular style among Pascal programmers. It’s not a widely used language today, but lots of developers cut their teeth on it a decade or three ago when drainpipe trousers, trilby hats, and black-and- white print T-shirts were the latest in fashion (or at least, they were in parts of Europe). And, by no coincidence whatsoever, Anders Hejlsberg (a key figure in the C# design team) also designed Borland’s Turbo Pascal. As for camel casing, that name comes from the fact that uppercase letters only ever appear in the middle of the name, meaning you get one or more humps in the middle, like a camel. There’s a wrinkle in these conventions. Acronyms generally get treated as though they are words, so if you had a class for an RGB color you might call it ColorRgb, and a color with an alpha channel might be ColorArgb. (The .NET Framework class libraries include types that refer to Argb, and people often mistakenly think that the “Arg” is short for “argument” rather than Alpha, Red, Green, and Blue.) There’s an exception to this exception: two-letter acronyms are usually capitalized. So a person’s intelligence quotient might be recorded as PersonIQ. These naming conventions are optional, but strongly recommended to help people understand your code. MSDN offers an extensive set of guidelines for these sorts of conventions at http://msdn.microsoft.com/library/ms229042. Defining Classes | 65 If we create an instance of this class, we could use this Identifier property to get and set its identifier. Example 3-3 shows this in a modified version of the Main function in our Program.cs file. Example 3-3. Using the Plane class’s property static void Main(string[] args) { Plane someBoeing777 = new Plane(); someBoeing777.Identifier = "BA0049"; Console.WriteLine( "Your plane has identifier {0}", someBoeing777.Identifier); // Wait for the user to press a key, so // that we can see what happened Console.ReadKey(); } But wait! If you try to compile this, you end up with an error message: 'Plane.Identifier' is inaccessible due to its protection level What’s that all about? Protection Levels Earlier, we mentioned that one of the objectives of good design is encapsulation: hiding the implementation details so that other developers can use our objects without relying on (or knowing about) how they work. As the error we just saw in Example 3-3 shows, a class’s members are hidden by default. If we want them to be visible to users of our class, we must change their protection level. Every entity that we declare has its own protection level, whether we specify it or not. A class, for example, has a default protection level called internal. This means that it can only be seen by other classes in its own assembly. We’ll talk a lot more about assemblies in Chapter 15. For now, though, we’re only using one assembly (our ex- ample application itself), so we can leave the class at its default protection level. While classes default to being internal, the default protection level for a class member (such as a property) is private. This means that it is only accessible to other members of the class. To make it accessible from outside the class, we need to change its protec- tion level to public, as Example 3-4 shows. Example 3-4. Making a property public class Plane { public string Identifier { 66 | Chapter 3: Abstracting Ideas with Classes and Structs get; set; } } Now when we compile and run the application, we see the correct output: Your plane has identifier BA0049 Notice how this is an opt-in scheme. If you don’t do anything to the contrary, you get the lowest sensible visibility. Your classes are visible to any code inside your assembly, but aren’t accessible to anyone else; a class’s properties and methods are only visible inside the class, unless you explicitly choose to make them more widely accessible. When different layers specify different protection, the effective accessibility is the low- est specified. For example, although our property has public accessibility, the class of which it is a member has internal accessibility. The lower of the two wins, so the Identifier property is, in practice, only accessible to code in the same assembly. It is a good practice to design your classes with the smallest possible public interface (part of something we sometimes call “minimizing the surface area”). This makes it easier for clients to understand how they’re supposed to be used and often cuts down on the amount of testing you need to do. Having a clean, simple public API can also improve the security characteristics of your class framework, because the larger and more complex the API gets, the harder it generally gets to spot all the possible lines of attack. That being said, there’s a common misconception that accessibility modifiers “secure” your class, by preventing people from accessing private members. Hence this warning: It is important to recognize that these protection levels are a convenient design constraint, to help us structure our applications properly. They are not a security feature. It’s possible to use the reflection features de- scribed in Chapter 17 to circumvent these constraints and to access these supposedly hidden details. To finish this discussion, you should know that there are two other protection levels available to us—protected and protected internal—which we can use to expose (or hide) members to developers who derive new classes from our class without making the members visible to all. But since we won’t be talking about derived classes until Chapter 4, we’ll defer the discussion of these protection levels until then. We can take advantage of protection in our Plane class. A plane’s identifier shouldn’t change mid-flight, and it’s a good practice for code to prevent things from happening that we know shouldn’t happen. We should therefore add that constraint to our class. Fortunately, we have the ability to change the accessibility of the getter and the setter individually, as Example 3-5 shows. (This is one reason the property syntax makes use declare the get and set explicitly—it gives us a place to put the protection level.) Defining Classes | 67 Example 3-5. Making a property setter private class Plane { public string Identifier { get; private set; } } Compiling again, we get a new error message: The property or indexer 'Plane.Identifier' cannot be used in this context because the set accessor is inaccessible The problem is with this bit of code from Example 3-3: someBoeing777.Identifier = "BA0049"; We’re no longer able to set the property, because we’ve made the setter private (which means that we can only set it from other members of our class). We wanted to prevent the property from changing, but we’ve gone too far: we don’t even have a way of giving it a value in the first place. Fortunately, there’s a language feature that’s perfect for this situation: a constructor. Initializing with a Constructor A constructor is a special method which allows you to perform some “setup” when you create an instance of a class. Just like any other method, you can provide it with pa- rameters, but it doesn’t have an explicit return value. Constructors always have the same name as their containing class. Example 3-6 adds a constructor that takes the plane’s identifier. Because the construc- tor is a member of the class, it’s allowed to use the Identifier property’s private setter. Example 3-6. Defining a constructor class Plane { public Plane(string newIdentifier) { Identifier = newIdentifier; } public string Identifier { get; private set; } } 68 | Chapter 3: Abstracting Ideas with Classes and Structs Notice how the constructor looks like a standard method declaration, except that since there’s no need for a return type specifier, we leave that out. We don’t even write void, like we would for a normal method that returns nothing. And it would be weird if we did; in a sense this does return something—the newly created Plane—it just does so implicitly. What sort of work should you do in a constructor? Opinion is divided on the subject— should you do everything required to make the object ready to use, or the minimum necessary to make it safe? The truth is that it is a judgment call—there are no hard and fast rules. Developers tend to think of a constructor as being a relatively low-cost op- eration, so enormous amounts of heavy lifting (opening files, reading data) might be a bad idea. Getting the object into a fit state for use is a good objective, though, because requiring other functions to be called before the object is fully operational tends to lead to bugs. We need to update our Main function to use this new constructor and to get rid of the line of code that was setting the property, as Example 3-7 shows. Example 3-7. Using a constructor static void Main(string[] args) { Plane someBoeing777 = new Plane("BA0049"); Console.WriteLine( "Your plane has identifier {0}", someBoeing777.Identifier); Console.ReadKey(); } Notice how we pass the argument to the constructor inside the parentheses, in much the same way that we pass arguments in a normal method call. If you compile and run that, you’ll see the same output as before—but now we have an identifier that can’t be changed by users of the object. Be very careful when you talk about properties that “can’t be changed” because they have a private setter. Even if you can’t set a property, you may still be able to modify the state of the object referred to by that property. The built-in string type happens to be immune to that be- cause it is immutable (i.e., it can’t be changed once it has been created), so making the setter on a string property private does actually prevent clients from changing the property, but most types aren’t like that. Speaking of properties that might need to change, our specification requires us to know the speed at which each plane is traveling. Sadly, our specification didn’t mention the units in which we were expected to express that speed. Let’s assume it is miles per hour, Defining Classes | 69 and add a suitable property. We’ll use the floating-point double data type for this. Example 3-8 shows the code to add to Plane. Example 3-8. A modifiable speed property public double SpeedInMilesPerHour { get; set; } If we were to review this design with the customer, they might point out that while they have some systems that do indeed want the speed in miles per hour the people they liaise with in European air traffic control want the speed in kilometers per hour. To avoid confusion, we will add another property so that they can get or set the speed in the units with which they are familiar. Example 3-9 shows a suitable property. Example 3-9. Property with code in its get and set public double SpeedInKilometersPerHour { get { return SpeedInMilesPerHour * 1.609344; } set { SpeedInMilesPerHour = value / 1.609344; } } We’ve done something different here—rather than just writing get; and set; we’ve provided code for these accessors. This is another reason we have to declare the acces- sors explicitly—the C# compiler needs to know whether we want to write a custom property implementation. We don’t want to use an ordinary property in Example 3-9, because our SpeedInKilo metersPerHour is not really a property in its own right—it’s an alternative representation for the information stored in the SpeedInMilesPerHour property. If we used the normal property syntax for both, it would be possible to set the speed as being both 100 mph and 400 km/h, which would clearly be inconsistent. So instead we’ve chosen to im- plement SpeedInKilometersPerHour as a wrapper around the SpeedInMilesPerHour property. If you look at the getter, you’ll see that it returns a value of type double. It is equivalent to a function with this signature: public double get_SpeedInKilometersPerHour() 70 | Chapter 3: Abstracting Ideas with Classes and Structs The setter seems to provide an invisible parameter called value, which is also of type double. So it is equivalent to a method with this signature: public void set_SpeedInKilometersPerHour(double value) This value parameter is a contextual keyword—C# only considers it to be a keyword in property or event accessors. (Events are described in Chapter 5.) This means you’re allowed to use value as an identifier in other contexts—for example, you can write a method that takes a pa- rameter called value. You can’t do that with other keywords—you can’t have a parameter called class, for example. This is a very flexible system indeed. You can provide properties that provide real stor- age in the class to store their data, or calculated properties that use any mechanism you like to get and/or set the values concerned. This choice is an implementation detail hidden from users of our class—we can switch between one and the other without changing our class’s public face. For example, we could switch the implementation of these speed properties around so that we stored the value in kilometers per hour, and calculated the miles per hour—Example 3-10 shows how these two properties would look if the “real” value was in km/h. Example 3-10. Swapping over the real and calculated properties public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / 1.609344; } set { SpeedInKilometersPerHour = value * 1.609344; } } public double SpeedInKilometersPerHour { get; set; } As far as users of the Plane class are concerned, there’s no discernible difference between the two approaches—the way in which properties work is an encapsulated implemen- tation detail. Example 3-11 shows an updated Main function that uses the new prop- erties. It neither knows nor cares which one is the “real” one. Defining Classes | 71 Example 3-11. Using the speed properties static void Main(string[] args) { Plane someBoeing777 = new Plane("BA0049"); someBoeing777.SpeedInMilesPerHour = 150.0; Console.WriteLine( "Your plane has identifier {0}, " + "and is traveling at {1:0.00}mph [{2:0.00}kph]", someBoeing777.Identifier, someBoeing777.SpeedInMilesPerHour, someBoeing777.SpeedInKilometersPerHour); someBoeing777.SpeedInKilometersPerHour = 140.0; Console.WriteLine( "Your plane has identifier {0}, " + "and is traveling at {1:0.00}mph [{2:0.00}kph]", someBoeing777.Identifier, someBoeing777.SpeedInMilesPerHour, someBoeing777.SpeedInKilometersPerHour); Console.ReadKey(); } Although our public API supports two different units for speed while successfully keeping the implementation for that private, there’s something unsatisfactory about that implementation. Our conversion relies on a magic number (1.609344) that appears repeatedly. Repetition impedes readability, and is prone to typos (I know that for a fact. I’ve typed it incorrectly once already this morning while preparing the example!) There’s an important principle in programming: don’t repeat yourself (or dry, as it’s sometimes abbreviated). Your code should aim to express any single fact or concept no more than once, because that way, you only need to get it right once. It would be much better to put this conversion factor in one place, give it a name, and refer to it by that instead. We can do that by declaring a field. Fields: A Place to Put Data A field is a place to put some data of a particular type. There’s no option to add code like you can in a property—a field is nothing more than data. Back before C# 3.0 the compiler didn’t let us write just get; and set;—we always had to write properties with code as in Example 3-9, and if we wanted a simple property that stored a value, we had to provide a field, with code such as Example 3-12. 72 | Chapter 3: Abstracting Ideas with Classes and Structs Example 3-12. Writing your own simple property // Field to hold the SpeedInMilesPerHour property's value double speedInMilesPerHourValue; public double SpeedInMilesPerHour { get { return speedInMilesPerHourValue; } set { speedInMilesPerHourValue = value; } } When you write just get; and set; as we did in Example 3-8, the C# compiler generates code that’s more or less identical to Example 3-12, except it gives the field a peculiar name to prevent us from accessing it directly. (These compiler-generated properties are called auto properties.) So, if we want to store a value in an object, there’s always a field involved, even if it’s a hidden one provided automatically by the compiler. Fields are the only class members that can hold information—properties are really just methods in disguise. As you can see, a field declaration looks similar to the start of a property declaration. There’s the type (double), and a name. By convention, this name is camelCased, to make fields visibly different from properties. (Some developers like to distinguish fields further by giving them a name that starts with an underscore.) We can modify a field’s protection level if we want, but, conventionally, we leave all fields with the default private accessibility. That’s because a field is just a place for some data, and if we make it public, we lose control over the internal state of our object. Properties always involve some code, even if it’s generated automatically by the com- piler. We can use private backing fields as we wish, or calculate property values any way we like, and we’re free to modify the implementation without ever changing the public face of the class. But with a field, we have nowhere to put code, so if we decide to change our implementation by switching from a field to a calculated value, we would need to remove the field entirely. If the field was part of the public contract of the class, that could break our clients. In short, fields have no innate capacity for encapsulation, so it’s a bad idea to make them public. Example 3-13 shows a modified version of the Plane class. Instead of repeating the magic number for our speed conversion factor, we declare a single field initialized to the required value. Not only does this mean that we get to state the conversion value just once, but we’ve also been able to give it a descriptive name—in the conversions, it’s now obvious that we’re multiplying and dividing by the number of kilometers in a mile, even if you happen not to have committed the conversion factor to memory. Defining Classes | 73 Example 3-13. Storing the conversion factor in a field class Plane { // Constructor with a parameter public Plane(string newIdentifier) { Identifier = newIdentifier; } public string Identifier { get; private set; } double kilometersPerMile = 1.609344; public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / kilometersPerMile; } set { SpeedInKilometersPerHour = value * kilometersPerMile; } } public double SpeedInKilometersPerHour { get; set; } } Notice how we’re able to initialize the field to a default value right where we declare it, by using the = operator. (This sort of code is called, predictably enough, a field initial- izer.) Alternatively, we could have initialized it inside a constructor, but if the default is a constant value, it is conventional to set it at the point of declaration. What about the first example of a field that we saw—the one we used as the backing data for a property in Example 3-12? We didn’t explicitly initialize it. In some other languages that would be a ghastly mistake. (Failure to initialize fields correctly is a major source of bugs in C++, for example.) Fortunately, the designers of .NET decided that the trade-off between performance and robustness wasn’t worth the pain, and kindly initialize all fields to a default value for us—numeric fields are set to zero and fields of other types get whatever the nearest equivalent of zero is. (Boolean fields are initialized to false, for example.) 74 | Chapter 3: Abstracting Ideas with Classes and Structs There’s also a security reason for this initialization. Because a new ob- ject’s memory is always zeroed out before we get to see it, we can’t just allocate a whole load of objects and then peer at the “uninitialized” values to see if anything interesting was left behind by the last object that used the same memory. Defining a field for our scale factor is an improvement, but we could do better. Our 1.609344 isn’t ever going to change. There are always that many kilometers per mile, not just for this instance of a Plane, but for any Plane there ever will be. Why allocate the storage for the field in every single instance? Wouldn’t it be better if we could define this value just once, and not store it in every Plane instance? Fields Can Be Fickle, but const Is Forever C# provides a mechanism for declaring that a field holds a constant value, and will never, ever change. You use the const modifier, as Example 3-14 shows. Example 3-14. Defining a constant value const double kilometersPerMile = 1.609344; The platform now takes advantage of the fact that this can never change, and allocates storage for it only once, no matter how many instances of Plane you new up. Handy. This isn’t just a storage optimization, though. By making the field const, there’s no danger that someone might accidentally change it for some reason inside another func- tion he’s building in the class—the C# compiler prevents you from assigning a value to a const field anywhere other than at the point of declaration. In general, when we are developing software, we’re trying to make it as easy as possible for other developers (including our “future selves”) to do the right thing, almost by accident. You’ll often hear this approach called “designing for the pit of success.” The idea is that people will fall into doing the right things because of the choices you’ve made. Some aspects of an object don’t fit well as either a normal modifiable field or a constant value. Take the plane’s identifier, for example—that’s fixed, in the sense that it never changes after construction, but it’s not a constant value like kilometersPerMile. Dif- ferent planes have different identifiers. .NET supports this sort of information through read-only properties and fields, which aren’t quite the same as const. Defining Classes | 75 Read-only Fields and Properties In Example 3-5, we made our Plane class’s Identifier property private. This prevented users of our class from setting the property, but our class is still free to shoot itself in the foot. Suppose a careless developer added some code like that in Example 3-15, which prints out messages in the SpeedInMilesPerHour property perhaps in order to debug some problem he was investigating. Example 3-15. Badly written debugging code public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / kilometersPerMile; } set { Identifier += ": speed modified to " + value; Console.WriteLine(Identifier); SpeedInKilometersPerHour = value * kilometersPerMile; } } The first time someone tries to modify a plane’s SpeedInMilesPerHour this will print out a message that includes the identifier, for example: BA0048: speed modified to 400 Unfortunately, the developer who wrote this clearly wasn’t the sharpest tool in the box—he used the += operator to build that debug string, which will end up modifying the Identifier property. So, the plane now thinks its identifier is that whole text, in- cluding the part about the speed. And if we modified the speed again, we’d see: BA0048: speed modified to 400: speed modified to 380 While it might be interesting to see the entire modification history, the fact that we’ve messed up the Identifier is bad. Example 3-15 was able to do this because the SpeedInMilesPerHour property is part of the Plane class, so it can still use the private setter. We can fix this (up to a point) by making the property read-only—rather than merely making the setter private, we can leave it out entirely. However, we can’t just write the code in Example 3-16. Example 3-16. The wrong way to define a read-only property class Plane { // Wrong! public string Identifier { get; } 76 | Chapter 3: Abstracting Ideas with Classes and Structs ... } That won’t work because there’s no way we could ever set Identifier—not even in the constructor. Auto properties cannot be read-only, so we must write a getter with code. Example 3-17 will compile, although as we’re about to see, the job’s not done yet. Example 3-17. A better, but incomplete, read-only property class Plane { public Plane(string newIdentifier) { _identifier = newIdentifier; } public string Identifier { get { return _identifier; } } private string _identifier; ... } This turns out to give us two problems. First, the original constructor from Exam- ple 3-6 would no longer compile—it set Identifier, but that’s now read-only. That was easy to fix, though—Example 3-17 just sets the explicit backing field we’ve added. More worryingly, this hasn’t solved the original problem—the developer who wrote the code in Example 3-15 has “cleverly” realized that he can “fix” his code by doing exactly the same thing as the constructor. As Example 3-18 shows he has just used the _identifier field directly. Example 3-18. “Clever” badly written debugging code public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / kilometersPerMile; } set { _identifier += ": speed modified to " + value; Console.WriteLine(Identifier); SpeedInKilometersPerHour = value * kilometersPerMile; } } That seemed like a long journey for no purpose. However, we can fix this problem— we can modify the backing field itself to be read-only, as shown in Example 3-19. Defining Classes | 77 Example 3-19. A read-only field private readonly string _identifier; That will foil the developer who wrote Example 3-15 and Example 3-18. But doesn’t it also break our constructor again? In fact, it doesn’t: read-only fields behave differently from read-only properties. A read-only property can never be modified. A read-only field can be modified, but only by a constructor. Since read-only fields only become truly read-only after construction completes, it makes them perfect for properties that need to be able to be different from one instance to another, but which need to be fixed for the lifetime of an instance. Before we move on from const and readonly fields, there’s another property our Plane needs for which const seems like it could be relevant, albeit in a slightly different way. In addition to monitoring the speed of an aircraft, we also need to know whether it is approaching or heading away from the airport. We could represent that with a bool property called something like IsApproaching (where true would mean that it was approaching, and false would, by implication, indicate that it was heading away). That’s a bit clumsy, though. You can often end up having to negate Boolean properties—you might need to write this sort of thing: if (!plane.IsApproaching) { ... } That reads as “if not plane is approaching” which sounds a bit awkward. We could go with: if (somePlane.IsApproaching == false) { ... } That’s “if is approaching is false” which isn’t much better. We could offer a second, calculated property called IsNotApproaching, but our code is likely to be simpler and easier to read (and therefore likely to contain fewer bugs) if, instead of using bool, we have a Direction property whose value could somehow be either Approaching or Leaving. We’ve just seen a technique we could use for that. We could create two constant fields of any type we like (int, for example), and a property of type int called Direction (see Example 3-20). Example 3-20. Named options with const int class Plane { public const int Approaching = 0; public const int Leaving = 1; // ... public int Direction { get; set; } } 78 | Chapter 3: Abstracting Ideas with Classes and Structs This lets us write code that reads a bit more naturally than it would if we had used just true and false: someBoeing777.Direction = Plane.Approaching; if (someAirbusA380.Direction == Plane.Leaving) { /* Do something */ } But there’s one problem: if our Direction property’s type is int, there’s nothing to stop us from saying something like: someBoeing777.Direction = 72; This makes no sense, but the C# compiler doesn’t know that—after all, we told it the property’s type was int, so how’s it supposed to know that’s wrong? Fortunately, the designers of C# have thought of this, and have given us a kind of type for precisely this situation, called an enum, and it turns out to be a much better solution for this than const int. Related Constants with enum The enum† keyword lets us define a type whose values can be one of a fixed set of possibilities. Example 3-21 declares an enum for our Direction property. You can add this to an existing source file, above or below the Plane class, for example. Alternatively, you could add a whole new source file to the project, although Visual Studio doesn’t offer a file template for enum types, so either you’d have to add a new class and then change the class keyword to enum, or you could use the Code File template to add a new, empty source file. Example 3-21. Direction enum enum DirectionOfApproach { Approaching, Leaving } This is similar in some respects to a class declaration. We can optionally begin with a protection level but if, like Example 3-21, we omit that, we get internal protection by default. Then there’s the enum specifier itself, followed by the name of the type, which by convention we PascalCase. Inside the braces, we declare the members, again using PascalCasing. Notice that we use commas to separate the list of constants—this is where the syntax starts to part company with class. Unusually, the members are pub- licly accessible by default. That’s because an enum has no behavior, and so there are no implementation details—it’s just a list of named values, and those need to be public for the type to serve any useful purpose. † It’s short for “enumeration,” by the way. So it’s often pronounced “e-noom” or, depending on where you’re from, “e-nyoom.” However, some developers (and one of the authors) ignore the etymology and pronounce it “ee numb” because that’s how it looks like it should sound. Related Constants with enum | 79 Notice that we’ve chosen to call this DirectionOfApproach, and not the plural DirectionsOfApproach. By convention, we give enum types a sin- gular name even though they usually contain a list. This makes sense because when you use named entries from an enumeration, you use them one at a time, and so it would look odd if the type name were plural. Obviously, there won’t be any technical consequences for break- ing this convention, but following it helps make your code consistent with the .NET Framework class libraries. We can now declare our Direction property, using the enumeration instead of an in- teger. Example 3-22 shows the property to add to the Plane class. Example 3-22. Property with enum type public DirectionOfApproach Direction { get; set; } There are some optional features we can use in an enum declaration. Example 3-23 uses these, and they provide some insight into how enum types work. Example 3-23. Explicit type and values for enum enum DirectionOfApproach : int { Approaching = 0, Leaving = 1 } In this declaration, we have explicitly specified the governing type for the enumeration. This is the type that stores the individual values for an enumeration, and we specify it with a colon and the type name. By default, it uses an int (exactly as we did in our original const-based implementation of this property), so we’ve not actually changed anything here; we’re just being more explicit. The governing type must be one of the built-in integer types: byte, sbyte, short, ushort, uint, long, or ulong. Example 3-23 also specifies the numbers to use for each named value. As it happens, if you don’t provide these numbers, the first member is assigned the value 0, and we count off sequentially after that, so again, this example hasn’t changed anything, it’s just showing the values explicitly. We could, if we wanted, specify any value for any particular member. Maybe we start from 10 and go up in powers of 2. And we’re also free to define duplicates, giving the same value several different names. (That might not be useful, but C# won’t stop you.) We normally leave all these explicit specifiers off, and accept the defaults. However, the sidebar on the next page describes a scenario in which you would need to control the numbers. 80 | Chapter 3: Abstracting Ideas with Classes and Structs Bit Fields with [Flags] You can create a special kind of enum called a [Flags] enum, also known as a bit field. A bit field is just an ordinary numeric value used in a particular way. When you view a bit field value in binary, each bit represents a particular setting. For example, we could define a bit field to represent the toppings on a bowl of ice cream. We might use the least significant bit to indicate whether a chocolate sauce topping is required. And we could use a different bit to indicate whether chocolate sprinkles are required. The thing that makes bit field enum types different from normal ones is that you can use any combination of values. Because each value gets a whole bit of the number to itself, you can choose for that bit to be either 0 or 1 independently of the value of any other bits. You indicate that your enum works this way by annotating it with a [Flags] attribute, and specifying the values of the members to correspond to the relevant bit patterns. (Actually, the [Flags] attribute turns out to be optional—the compiler ignores it, and lets you use any enum as though it were a bit field. The .NET Framework only uses the attribute to work out how to convert enumeration values to text. However, it’s a useful signpost to tell other developers how your enum is meant to be used.) Typically, you define a name for each bit, and you can also name some common combinations: [Flags] enum Toppings { None = 0x00, // Special zero value ChocolateSauce = 0x01, ToffeeSauce = 0x02, ChocolateSprinkles = 0x04, Chocoholic = 0x05, // Combined value, sets 2 bits Greedy = 0x07 // Everything! } We’re using hexadecimal representations because it’s easier to relate them to the binary values—each hex digit corresponds exactly to four binary digits. We can combine the values together using the | operator (binary OR), for example: // (011) Toppings saucy = Toppings.ChocolateSauce | Toppings.ToffeeSauce; We can use the binary AND operator (&) to see whether a particular flag has been set: static bool DoYouWantChocolateSauceWithThat(Toppings t) { return (t & Toppings.ChocolateSauce) != 0; } When defining bit fields, you might not want to allow certain combinations. For ex- ample, you might reject the saucy combination, requiring customers to pick, at most, one kind of sauce. Unfortunately, there are no language or platform mechanisms for enforcing that kind of constraint, so you’d need to write code to check for illegal com- binations in any method that accepted arguments of this type. (Or you could consider an alternative design that does not use an enum at all.) Related Constants with enum | 81 If you don’t specify explicit values, the first item in your list is effectively the default value for the enum (because it corresponds to the zero value). If you provide explicit values, be sure to define a value that corresponds to zero—if you don’t, fields using your type will default to a value that’s not a valid member of the enum, which is not desirable. We can now access the enumeration property like this: someBoeing777.Direction = DirectionOfApproach.Approaching; We’ve clearly made some progress with our Plane class, but we’re not done yet. We have a read-only property for its Identifier. We can store the speed, which we can get and set using two different properties representing different units, using a const field for the conversion factor. And we know the direction, which will be either the Approach ing or the Leaving member of an enum. We still need to store the aircraft’s position. According to the specification, we’ve got two polar coordinates (an angle and a distance) for its position on the ground, and another value for its height above sea level. We’re likely to need to do a lot of calculations based on this position information. Every time we want to create a function to do that, we’d need three parameters per point, which seems overly complex. (And error-prone—it’d be all too easy to inadvertently pass two numbers from one position, and a third number from a different position.) It would be nicer if we could wrap the numbers up into a single, lightweight, “3D point” type that we can think of in the same kind of way we do int or double—a basic building block for other classes to use with minimum overhead. This is a good candidate for a value type. Value Types and Reference Types So far, we’ve been building a class. When creating an instance of the class, we stored it in a named variable, as Example 3-24 shows. Example 3-24. Storing a reference in a variable Plane someBoeing777 = new Plane("BA0049"); someBoeing777.Direction = DirectionOfApproach.Approaching; We can define another variable with a different name, and store a reference to the same plane in that new variable, as shown in Example 3-25. Example 3-25. Copying a reference from one variable to another Plane theSameBoeing777ByAnotherName = someBoeing777; 82 | Chapter 3: Abstracting Ideas with Classes and Structs If we change a property through one variable, that change will be visible through the other. Example 3-26 modifies our plane’s Direction property through the second var- iable, but then reads it through the first variable, verifying that they really are referring to the same object. Example 3-26. Using one object through two variables theSameBoeing777ByAnotherName.Direction = DirectionOfApproach.Leaving; if (someBoeing777.Direction == DirectionOfApproach.Leaving) { Console.WriteLine("Oh, they are the same!"); } As Shakespeare might have said, if only he’d found his true vocation as a C# developer: That which we call someBoeing777 By any other name would smell as sweet. Assuming you like the smell of jet fuel. When we define a type using class, we always get this behavior—our variables behave as references to an underlying object. We therefore call a type defined as a class a reference type. It’s possible for a reference type variable to be in a state where it isn’t referring to any object at all. C# has a special keyword, null, to represent this. You can set a variable to null, or you can pass null as an argument to a method. And you can also test to see if a field, variable, or argument is equal to null in an if statement. Any field whose type is a reference type will automatically be initialized to null before the constructor runs, in much the same way as numeric fields are initialized to zero. The enum we declared earlier and the built-in numeric types (int, double) behave dif- ferently, though, as Example 3-27 illustrates. Example 3-27. Copying values, not references int firstInt = 3; int secondInt = firstInt; secondInt = 4; if (firstInt != 4) { Console.WriteLine("Well. They're not the same at all."); } When we assign firstInt to secondInt, we are copying the value. In this case, the var- iables hold the actual value, not a reference to a value. We call types that behave this way value types. Value Types and Reference Types | 83 People often refer to reference types as being allocated “on the heap” and value types “on the stack.” C++ programmers will be familiar with these concepts, and C++ pro- vided one syntax in the language to explicitly create items on the stack (a cheap form of storage local to a particular scope), and a different syntax for working on the heap (a slightly more expensive but sophisticated form of storage that could persist beyond the current scope). C# doesn’t make that distinction in its syntax, because the .NET Framework itself makes no such distinction. These aspects of memory management are completely opaque to the developer, and it is actively wrong to think of value types as being always allocated on a stack. For people familiar with C++ this can take a while to get used to, especially as the myth is perpetuated on the Web, in the MSDN documentation and elsewhere. (For example, at the time of this writing, http://msdn.microsoft.com/library/aa288471 states that structs are created on the stack, and while that happens to be true of the ones in that example when running against the current version of .NET, it would have been helpful if the page had mentioned that it’s not always true. For example, if a class has a field of value type, that field doesn’t live on the stack—it lives inside the object, and in all the versions of .NET released so far, objects live on the heap.) The important difference for the C# developer between these two kinds of types is the one of reference versus copy semantics. As well as understanding the difference in behavior, you also need to be aware of some constraints. To be useful, a value type should be: • Immutable • Lightweight Something is immutable if it doesn’t change over time. So, the integer 3 is immutable. It doesn’t have any internal workings that can change its “three-ness”. You can replace the value of an int variable that currently contains a 3, by copying a 4 into it, but you can’t change a 3 itself. (Unlike, say, a particular Plane object, which has a Direction property that you can change anytime you like without needing to replace the whole Plane.) There’s nothing in C# that stops you from creating a mutable value type. It is just a bad idea (in general). If your type is mutable, it is prob- ably safer to make it a reference type, by declaring it as a class. Mutable value types cause problems because of the copy semantics—if you mod- ify a value, it’s all too easy to end up modifying the wrong one, because there may be many copies. 84 | Chapter 3: Abstracting Ideas with Classes and Structs It should be fairly apparent that a value type also needs to be pretty lightweight, because of all that copying going on. Every time you pass it into a function, or assign it to a variable, a copy is made. And copies are generally the enemy of good performance. If your value type consists of more than two or three of the built-in types, it may be getting too big. These constraints mean it is very rare that you will actually want to declare a value type yourself. A lot of the obviously useful ones you might want are already defined in the .NET Framework class libraries (things like 2D points, times, and dates). Custom value types are so rare that it was hard to come up with a useful example for this book that wasn’t already provided in the class libraries. (If you were wondering why our example application represents aircraft positions in such an idiosyncratic fashion, this is the reason.) But that doesn’t mean you should never, ever declare a value type. Value types can have performance benefits when used in arrays (although as with most performance issues, this is not entirely clear-cut), and the immutability and copy semantics can make them safer when passing them in to functions—you won’t normally introduce side effects by working with a value type because you end up using a copy, rather than modifying shared data that other code might be relying on. Our polar 3D point seems to comply with the requirements. Any given point is just that: a specific point in 3D space—a good candidate for immutability. (We might want to move a plane to a different point, but we can’t change what a particular point means.) It is also no more than three doubles in size, which is small enough for copy semantics. Example 3-28 shows our declaration of this type, which we can add to our project. (As with enum, Visual Studio doesn’t offer a template for value types. Again, we can use the Class template, replacing the class with the code we want.) Example 3-28. A value type struct PolarPoint3D { public PolarPoint3D(double distance, double angle, double altitude) { Distance = distance; Angle = angle; Altitude = altitude; } public double Distance { get; private set; } public double Angle { get; private set; Value Types and Reference Types | 85 } public double Altitude { get; private set; } } If you think that it looks just like a class declaration, but using the struct keyword instead of class, you’d be right—these two kinds of types are very similar. However, if we try to compile it, we get an error on the first line of the constructor: The 'this' object cannot be used before all of its fields are assigned to So, although the basic syntax of a struct looks just like a class there are important differences. Remember that when you allocate an instance of a particular type, it is always initialized to some default value. With classes, all fields are initialized to zero (or the nearest equivalent value). But things work slightly differently with value types— we need to do slightly more work. Anytime we write a struct, C# automatically generates a default, parameterless con- structor that initializes all of our storage to zero, so if we don’t want to write any custom constructors, we won’t have any problems. (Unlike with a class, we aren’t allowed to replace the default constructor. We can define extra constructors, but the default con- structor is always present and we’re not allowed to write our own—see the sidebar on the next page for details.) Example 3-28 has hit trouble because we’re trying to provide an additional constructor, which initializes the properties to particular values. If we write a constructor in a struct, the compiler refuses to let us invoke any methods until we’ve initialized all the fields. (It doesn’t do the normal zero initialization for custom constructors.) This re- striction turns out to include properties, because get and set accessors are methods under the covers. So C# won’t let us use our properties until the underlying fields have been initialized, and we can’t do that because these are auto properties—the C# com- piler has generated hidden fields that we can only access through the properties. This is a bit of a chicken-and-egg bootstrapping problem! Fortunately, C# gives us a way of calling one of our constructors from another. We can use this to call the default constructor to do the initialization; then our constructor can set the properties to whatever values it wishes. We call the constructor using the this keyword, and the standard function calling syntax with any arguments enclosed in parentheses. As Example 3-29 shows, we can invoke the default constructor with an empty argument list. 86 | Chapter 3: Abstracting Ideas with Classes and Structs Value Types and Default Constructors Why aren’t we allowed to define a custom default constructor for a value type, given that we’re allowed to do that for a reference type? The short answer is that the speci- fication for the relevant behavior in the .NET Framework doesn’t let you. (The speci- fication in question is called the Common Language Infrastructure [CLI], incidentally.) The slightly longer answer is: for efficiency reasons. By mandating that the default constructor for any value type always initializes everything to zero, large arrays of value types can be constructed very cheaply, just by allocating the required amount of mem- ory and zeroing out the whole array in one step. And similarly, it simplifies the initial- ization of fields and variables—everything can be initialized to zero. Example 3-29. Calling one constructor from another public PolarPoint3D(double distance, double angle, double altitude) : this() { Distance = distance; Angle = angle; Altitude = altitude; } You add the call just before the opening brace for the body of the constructor, and prefix it with a colon. We can also use this technique to avoid writing common initi- alization code multiple times. Say we wanted to provide another utility constructor that just took the polar coordinates, and initialized the altitude to zero by default. Instead of repeating all the code from the first constructor, we could just add this extra con- structor to our definition for PolarPoint3D, as shown in Example 3-30. Example 3-30. Sharing common initialization code public PolarPoint3D(double distance, double angle) : this(distance, angle, 0) { } public PolarPoint3D( double distance, double angle, double altitude) : this() { Distance = distance; Angle = angle; Altitude = altitude; } Incidentally, this syntax for calling one constructor from another works equally well in classes, and is a great way of avoiding code duplication. Value Types and Reference Types | 87 Too Many Constructors, Mr. Mozart You should be careful of adding too many constructors to a class or struct. It is easy to lose track of which parameters are which, or to make arbitrary choices about which constructors you provide and which you don’t. For example, let’s say we wanted to add yet another constructor to PolarPoint3D that lets callers pass just the angle and altitude, initializing the distance to a default of zero, as Example 3-31 shows. Example 3-31. A constructor too far public PolarPoint3D( double altitude, double angle ) : this( 0, angle, altitude ) { } Even before we compile, we can see that there’s a problem—we happen to have added the altitude parameter so that it is the first in the list, and angle stays second. In our main constructor, the altitude comes after the angle. Because they are both just doubles, there’s nothing to stop you from accidentally passing the parameters “the wrong way round.” This is the exactly the kind of thing that surprises users of your class, and leads to hard-to-find bugs. But while inconsistent parameter ordering is bad design, it’s not a showstopper. However, when we compile, things get even worse. We get another error: Type 'PolarPoint3D' already defines a member called 'PolarPoint3D' with the same parameter types We have too many constructors. But how many is too many? Overloading When we define more than one member in a type with the same name (be it a con- structor or, as we’ll see later, a method) we call this overloading. Initially, we created two constructors (two overloads of the constructor) for Polar Point3D, and they compiled just fine. This is because they took different sets of param- eters. One took three doubles, the other two. In fact, there was also the third, hidden constructor that took no parameters at all. All three constructors took different num- bers of parameters, meaning there’s no ambiguity about which constructor we want when we initialize a new PolarPoint3D. The constructor in Example 3-31 seems different: the two doubles have different names. Unfortunately, this doesn’t matter to the C# compiler—it only looks at the types of the parameters, and the order in which they are declared. It does not use names for 88 | Chapter 3: Abstracting Ideas with Classes and Structs disambiguation. This should hardly be surprising, because we’re not required to pro- vide argument names when we call methods or constructors. If we add the overload in Example 3-31, it’s not clear what new PolarPoint3D(0, 0) would mean, and that’s why we get an error—we’ve got two members with the same name (PolarPoint3D—the constructor), and exactly the same parameter types, in the same order. Looking at overloaded functions will emphasize that it really is only the method name and the parameters that matter—a function’s return type is not considered to be a disambiguating aspect of the member for overload purposes. That means there’s nothing we can do about it: we’re going to have to get rid of this third constructor (just delete it); and while we’re in the code, we’ll finish up the dec- laration of the data portion of our Plane by adding a property for its position, shown in Example 3-32. Example 3-32. Using our custom value type for a property public PolarPoint3D Position { get; set; } Overloaded Methods and Default Named Parameters Just as with constructors, we can provide more than one method with the same name, but a different list of parameter types. It is, in general, a bad idea to provide two over- loads with the same name if they perform a semantically different operation (again— that’s the kind of thing that surprises developers using your class), so the most common reason for overloading is to provide several different ways to do something. We can provide users of our code with flexible methods that take lots of arguments to control different aspects of the code, and we can also provide developers that don’t need this flexibility with simpler options by providing overloads that don’t need as many arguments. Suppose we added a method to our Plane class enabling messages to be sent to aircraft. Perhaps in our first attempt we define a method whose signature looks like this: public void SendMessage(string messageText) But suppose that as the project progresses, we find that it would be useful to be able to delay transmission of certain messages. We could modify the SendMessage method so that it accepts an extra argument. There’s a handy type in the framework called TimeSpan which lets us specify duration. We could modify the method to make use of it: public void SendMessage(string messageText, TimeSpan delay) Alas! If we already had code in our project depending on the original signature, we’d start to see this compiler error: No overload for method 'SendMessage' takes '1' arguments Overloading | 89 We’ve changed the signature of that method, so all our clients are sadly broken. They need to be rewritten to use the new method. That’s not great. A better alternative is to provide both signatures—keep the old single-parameter con- tract around, but add an overload with the extra argument. And to ensure that the overloads behave consistently (and to avoid duplicating code) we can make the simpler method call the new method as its actual implementation. The old method was just the equivalent of calling the new method with a delay of zero, so we could replace it with the method shown in Example 3-33. This lets us provide the newly enhanced SendMessage, while continuing to support the old, simpler version. Example 3-33. Implementing one overload in terms of another public void SendMessage(string messageName) { SendMessage(messageName, TimeSpan.Zero); } (TimeSpan.Zero is a static field that returns a duration of zero.) Until C# 4.0 that’s as far as we could go. However, the C# designers noticed that a lot of member overloads were just like this one: facades over an über-implementation, with a bunch of parameters defaulted out to particular values. So they decided to make it easier for us to support multiple variations on the same method. Rather than writing lots of overloads, we can now just specify default values for a method’s arguments, which saves us typing a lot of boilerplate, and helps make our default choices more transparent. Let’s take out the single-parameter method overload we just added, and instead change the declaration of our multiparameter implementation, as shown in Example 3-34. Example 3-34. Parameter with default value public void SendMessage( string messageName, TimeSpan delay = default(TimeSpan)) Even though we’ve only got one method, which supports two arguments, code that tries to call it with a single argument will still work. That’s because default values can fill in for missing arguments. (If we tried to call SendMessage with no arguments at all, we’d get a compiler error, because there’s no default for the first argument here.) But it doesn’t end there. Say we had a method with four parameters, like this one: public void MyMethod( int firstOne, double secondInLine = 3.1416, string thirdHere = "The third parameter", TimeSpan lastButNotLeast = default(TimeSpan)) { // ... } 90 | Chapter 3: Abstracting Ideas with Classes and Structs If we want to call it and specify the first parameter (which we have to, because it has no default), and the third, but not the second or the fourth, we can do so by using the names of the parameters, like this: MyMethod(127, thirdHere: "New third parameter"); With just one method, we now have many different ways to call it—we can provide all the arguments, or just the first and second, or perhaps the first, second, and third. There are many combinations. Before named arguments and defaults were added in C# 4.0, the only way to get this kind of flexibility was to write an overload for each distinct combination. Under the Hood with Default Parameters Default and named parameters are very useful features, but we need to warn you of a subtle potential problem. Although they are more-or-less equivalent to providing a bunch of different function overloads, as far as the syntax for the caller goes, under the covers, they are implemented very differently. The compiler marks a parameter to indicate that it is optional using the OptionalAttri bute and there’s a DefaultParameterValueAttribute to specify a default value. These two attributes have been around for quite a while—they were originally added for the benefit of Visual Basic, long before C# started using them. (Attributes are discussed in Chapter 17.) When you call a method (or constructor), the C# compiler always emits a complete call—the compiled code passes a full set of arguments to the method, even if your source code left some arguments out. For example, in our Plane example, if you wrote: SendMessage("SomeMessage"); but the Plane class only has the method shown in Example 3-34, the compiler actually generates code equivalent to this: SendMessage("SomeMessage", default(TimeSpan)); In other words, it plugs in the default value at compile time. This means if you’re using some external library that uses default values, and a newer version of the library comes out that changes the default values for some method or constructor, your code won’t pick up those new values unless you recompile your code. There’s also a subtler problem you can run into. Some parts of the .NET Framework require you to provide a particular constructor overload. For example, it you write a custom control for WPF, and you want to use it from Xaml, it must have a default constructor. (WPF and Xaml are described in Chapter 20.) If all your constructors take parameters, then even if you provide default values for all the parameters, that’s not good enough. You can write, say, new MyControl() in C#, but only because the C# compiler is implicitly passing the missing values for you. Not everything in the world of .NET understands the concept of default arguments. (C# itself didn’t until version 4.0.) Sometimes only a genuine no-arguments constructor will do. Overloading | 91 This is not just limited to normal methods—you can use this same syntax to provide default values for parameters in your constructors, if you wish. Being forced to delete the extra constructor we tried to add back in Example 3-31 was a little disappointing—we’re constraining the number of ways users of our type can initialize it. Named arguments and default values have helped, but can we do more? Object Initializers Until C# 3.0, the only real solution to this was to write one or more factory methods. These are described in the sidebar below. But now we have another option. Factory Methods A factory method is a static method that builds a new object. There’s no formal support for this in C#, it’s just a common solution to a problem—a pattern, as popular idioms are often called in programming. We can get around the overload ambiguity problems by providing factory methods with different names. And the names can make it clear how we’re initializing the instance: public static PolarPoint3D FromDistanceAndAngle( double distance, double angle) { return new PolarPoint3D(distance, angle, 0); } public static PolarPoint3D FromAngleAndAltitude( double angle, double altitude) { return new PolarPoint3D(0, angle, altitude); } We rather like this approach, although some people frown on it as insufficiently dis- coverable. (Most developers aren’t expecting to find static methods that act rather like constructors, and if nobody finds these methods, we’re wasting our time in providing them.) However, this pattern is used all over the .NET Framework libraries—DateTime, TimeSpan, and Color are popular types that all use this technique. With C# 3.0 the language was extended to support object initializers—an extension to the new syntax that lets us set up a load of properties, by name, as we create our object instance. Example 3-35 shows how an object initializer looks when we use it in our Main function. 92 | Chapter 3: Abstracting Ideas with Classes and Structs Example 3-35. Using object initializers static void Main(string[] args) { Plane someBoeing777 = new Plane("BA0049") { Direction = DirectionOfApproach.Approaching, SpeedInMilesPerHour = 150 }; Console.WriteLine( "Your plane has identifier {0}," + " and is traveling at {1:0.00}mph [{2:0.00}kph]", // Use the property getter someBoeing777.Identifier, someBoeing777.SpeedInMilesPerHour, someBoeing777.SpeedInKilometersPerHour); someBoeing777.SpeedInKilometersPerHour = 140.0; Console.WriteLine( "Your plane has identifier {0}," + " and is traveling at {1:0.00}mph [{2:0.00}kph]", // Use the property getter someBoeing777.Identifier, someBoeing777.SpeedInMilesPerHour, someBoeing777.SpeedInKilometersPerHour); Console.ReadKey(); } Object initializers are mostly just a convenient syntax for constructing a new object and then setting some properties. Consequently, this only works with writable properties—you can’t use it for immutable types,‡ so this wouldn’t work with our PolarPoint3D. We still use the constructor parameter for the read-only Identifier property; but then we add an extra section in braces, between the closing parenthesis and the semicolon, in which we have a list of property assignments, separated by commas. What’s partic- ularly interesting is that the purpose of the constructor parameter is normally identifiable only by the value we happen to assign to it, but the object initializer is “self-documenting”—we can easily see what is being initialized to which values, at a glance. ‡ This is a slight oversimplification. In Chapter 8, we’ll encounter anonymous types, which are always immutable, and yet we can use object initializers with those. In fact, we are required to. But anonymous types are a special case. Object Initializers | 93 The job isn’t quite done yet, though. While there’s nothing technically wrong with using both the constructor parameter and the object initializer, it does look a little bit clumsy. It might be easier for our clients if we allow them to use a default, parameterless constructor, and then initialize all the members using this new syntax. As we’ll see in Chapter 6, we have other ways of enforcing invariants in the object state, and dealing with incorrect usages. Object initializers are certainly a more expressive syntax, and on the basis that self-documenting and transparent is better, we’re going to change how Plane works so that we can initialize the whole object with an object initializer. As with any design consideration, there is a counter argument. Some classes may be downright difficult to put into a “default” (zero-ish) state that isn’t actively dangerous. We’re also increasing the size of the public API by the changes we’re making—we’re adding a public setter. Here, we’ve decided that the benefits outweigh the disadvantages in this par- ticular case (although it’s really a judgment call; no doubt some devel- opers would disagree). First, as Example 3-36 shows, we’ll delete the special constructor from Plane, and then make Identifier an ordinary read/write property. We can also remove the _identifier backing field we added earlier, because we’ve gone back to using an auto property. Example 3-36. Modifying Plane to work better with object initializers class Plane { // Remove the constructor that we no longer require // public Plane(string newIdentifier) // { // Identifier = newIdentifier; // } public string Identifier { get; // remove the access modifier // to make it public set; } // ... } We can now use the object initializer syntax for all the properties we want to set. As Example 3-37 shows, this makes our code look somewhat neater—we only need one style of code to initialize the object. 94 | Chapter 3: Abstracting Ideas with Classes and Structs Example 3-37. Nothing but object initializer syntax Plane someBoeing777 = new Plane { Identifier = "BA0049", Direction = DirectionOfApproach.Approaching, SpeedInMilesPerHour = 150 }; Object initializer syntax provides one big advantage over offering lots of specialized constructors: people using your class can provide any combination of properties they want. They might decide to set the Position property inline in this object initializer too, as Example 3-38 does—if we’d been relying on constructors, default or named argu- ments wouldn’t have helped if there was no constructor available that accepted a Position. We’ve not had to provide an additional constructor overload to make this possible—developers using our class have a great deal of flexibility. Of course, this approach only makes sense if our type is able to work sensibly with default values for the properties in question. If you absolutely need certain values to be provided on initialization, you’re better off with constructors. Example 3-38. Providing an extra property Plane someBoeing777 = new Plane { Identifier = "BA0049", Direction = DirectionOfApproach.Approaching, SpeedInMilesPerHour = 150, Position = new PolarPoint3D(20, 180, 14500) }; So, we’ve addressed the data part of our Plane; but the whole point of a class is that it can encapsulate both state and operations. What methods are we going to define in our class? Defining Methods When deciding what methods a class might need, we generally scan our specifications or scenarios for verbs that relate to the object of that class. If we look back at the ATC system description at the beginning of this chapter, we can see several plane-related actions, to do with granting permissions to land and permissions to take off. But do we need functions on the Plane class to deal with that? Possibly not. It might be better to deal with that in another part of the model, to do with our ground control, runways, and runway management (that, you’ll be pleased to hear, we won’t be building). But we will periodically need to update the position of all the planes. This involves changing the state of the plane—we will need to modify its Position. And it’s a change of state whose details depend on the existing state—we need to take the direction and Defining Methods | 95 speed into account. This sounds like a good candidate for a method that the Plane class should offer. Example 3-39 shows the code to add inside the class. Example 3-39. A method public void UpdatePosition(double minutesToAdvance) { double hours = minutesToAdvance / 60.0; double milesMoved = SpeedInMilesPerHour * hours; double milesToTower = Position.Distance; if (Direction == DirectionOfApproach.Approaching) { milesToTower -= milesMoved; if (milesToTower < 0) { // We've arrived! milesToTower = 0; } } else { milesToTower += milesMoved; } PolarPoint3D newPosition = new PolarPoint3D( milesToTower, Position.Angle, Position.Altitude); } This method takes a single argument, indicating how much elapsed time the calculation should take into account. It looks at the speed, the direction, and the current position, and uses this information to calculate the new position. This code illustrates that our design is some way from being finished. We never change the altitude, which suggests that our planes are going to have a hard time reaching the ground. (Although since this code makes them stop moving when they get directly above the tower, they’ll probably reach the ground soon enough...) Apparently our initial spec- ification did not fully and accurately describe the problem our software should be solving. This will not come as astonishing news to anyone who has worked in the software industry. Clearly we need to talk to the client to get clarification, but let’s implement what we can for now. Notice that our code is able to use all of the properties—SpeedInMilesPerHour, Direction, and so on—without needing to qualify them with a variable. Whereas in Example 3-35 we had to write someBoeing777.SpeedInMilesPerHour, here we just write SpeedInMilesPerHour. Methods are meant to access and modify an object’s state, and so you can refer directly to any member of the method’s containing class. There’s one snag with that. It can mean that for someone reading the code, it’s not always instantly obvious when the code uses a local variable or argument, and when it uses some member of the class. Our properties use PascalCasing, while we’re using 96 | Chapter 3: Abstracting Ideas with Classes and Structs camelCasing for arguments and variables, which helps, but what it we wanted to access a field? Those conventionally use camelCasing too. That’s why some developers put an underscore in front of their field names—it makes it more obvious when we’re doing something with the object’s state. But there’s an alternative—a more explicit style, shown in Example 3-40. Example 3-40. Explicit member access public void UpdatePosition(double minutesToAdvance) { double hours = minutesToAdvance / 60; double milesMoved = this.SpeedInMilesPerHour * hours; double milesToTower = this.Position.Distance; if (this.Direction == DirectionOfApproach.Approaching) { milesToTower -= milesMoved; if (milesToTower < 0) { // We've arrived! milesToTower = 0; } } else { milesToTower += milesMoved; } PolarPoint3D newPosition = new PolarPoint3D( milesToTower, this.Position.Angle, this.Position.Altitude); } This is almost the same as Example 3-39, except every member access goes through a variable called this. But we’ve not defined any such variable—where did that come from? The UpdatePosition method effectively has an implied extra argument called this, and it’s the object on which the method has been invoked. So, if our Main method were to call someBoeing777.UpdatePosition(10), the this variable would refer to whatever ob- ject the Main method’s someBoeing777 variable referred to. Methods get a this argument by default, but they can opt out, because sometimes it makes sense to write methods that don’t apply to any particular object. The Main method of our Program class is one example—it has no this argument, because the .NET Framework doesn’t presume to create an object; it just calls the method and lets us decide what objects, if any, to create. You can tell a method has no this argument because it will be marked with the static keyword—you may recall from Chapter 2 that this means the method can be run without needing an instance of its defining type. Aside from our Main method, why might we not want a method to be associated with a particular instance? Well, one case comes to mind for our example application. Defining Methods | 97 There’s a rather important feature of airspace management that we’re likely to need to cope with: ensuring that we don’t let two planes hit each other. So, another method likely to be useful is one that allows us to check whether one plane is too close to another one, within some margin of error (say, 5,000 feet). And this method isn’t associated with any single plane: it always involves two planes. Now we could define a method on Plane that accepted another Plane as an argument, but that’s a slightly misleading design—it has a lack of symmetry which suggests that the planes play different roles, because you’re invoking the method on one while pass- ing in the other as an argument. So it would make more sense to define a static method—one not directly associated with any single plane—and to have that take two Plane objects. Declaring Static Methods We’ll add the method shown in Example 3-41 to the Plane class. Because it is marked static, it’s not associated with a single Plane, and will have no implicit this argument. Instead, we pass in both of the Plane objects we want to look at as explicit arguments, to emphasize the fact that neither of the objects is in any way more significant than the other in this calculation. Example 3-41. Detecting when Planes are too close public static bool TooClose(Plane first, Plane second, double minimumMiles) { double x1 = first.Position.Distance * Math.Cos(first.Position.Angle); double x2 = second.Position.Distance * Math.Cos(second.Position.Angle); double y1 = first.Position.Distance * Math.Sin(first.Position.Angle); double y2 = second.Position.Distance * Math.Sin(second.Position.Angle); double z1 = first.Position.Altitude / feetPerMile; double z2 = second.Position.Altitude / feetPerMile; double dx = x1 - x2; double dy = y1 - y2; double dz = z1 - z2; double distanceSquared = dx * dx + dy * dy + dz * dz; double minimumSquared = minimumMiles * minimumMiles; return distanceSquared < minimumSquared; } private const double feetPerMile = 5280; We’ve seen plenty of function declarations like this before, but we’ll quickly recap its anatomy. This one returns a bool to indicate whether we’re safe (true) or not (false). In its parameter list, we have the references to the two Plane objects, and a double for the margin of error (in miles). 98 | Chapter 3: Abstracting Ideas with Classes and Structs Because there’s no implicit this parameter, any attempt to use nonstatic members of the class without going through an argument or variable such as first and second in Example 3-41 will cause an error. This often catches people out when learning C#. They try adding a method to the Program class of a new program, and they forget to mark it as static (or don’t realize that they need to), and then are surprised by the error they get when attempting to call it from Main. Main is a static method, and like any static method, it cannot use nonstatic members of its contain- ing type unless you provide it with an instance. Example 3-41 performs some calculations to work out how close the planes are. The details aren’t particularly important here—we’re more interested in how this uses C# methods. But just for completeness, the method converts the position into Cartesian coordinates, and then calculates the sum of the squares of the differences of the coor- dinates in all three dimensions, which will give us the square of the distance between the two planes. We could calculate the actual distance by taking the square root, but since we only want to know whether or not we’re too close, we can just compare with the minimum distance squared. (Computers are much faster at squaring than they are at calculating square roots, so given that we could do it either way, we may as well avoid the square root.) Static Fields and Properties It isn’t just functions that we can declare as static. Fields and properties can be static, too. In fact, we’ve already seen a special kind of static field—the const value we defined for the conversion between miles and kilometers. There was only one conversion factor value, however many objects we instantiated. The only difference between a const field and a static field is that we can modify the static field. (Remember: the const field was immutable.) So, a static property or field effectively lets us get or set data associated with the class, rather than the object. No matter how many objects we create, we are always getting and setting the same value. Let’s look at a trivial illustration, shown in Example 3-42, to explore how it works, before we think about why we might want to use it. Example 3-42. Static state public class MyClassWithAStaticProperty { public static bool TrueOrFalse { get; set; } public void SayWhetherTrueOrFalse() Static Fields and Properties | 99 { Console.WriteLine("Object is {0}", TrueOrFalse); } } class Program { static void Main(string[] args) { // Create two objects MyClassWithAStaticProperty object1 = new MyClassWithAStaticProperty(); MyClassWithAStaticProperty object2 = new MyClassWithAStaticProperty(); // Check how the property looks to each object, // and accessed through the class name object1.SayWhetherTrueOrFalse(); object2.SayWhetherTrueOrFalse(); Console.WriteLine("Class is {0}", MyClassWithAStaticProperty.TrueOrFalse); // Change the value MyClassWithAStaticProperty.TrueOrFalse = true; // And see that it has changed everywhere object1.SayWhetherTrueOrFalse(); object2.SayWhetherTrueOrFalse(); Console.WriteLine("Class is {0}", MyClassWithAStaticProperty.TrueOrFalse); Console.ReadKey(); } } If you compile and run this code in a console application project, you’ll see the following output: Object is False Object is False Class is False Object is True Object is True Class is True This demonstrates that there’s clearly just the one piece of information here, no matter how many different object instances we may try to look at it through. But why might we want this kind of static, class-level data storage? The principal use for class-level data is to enforce the reality that there is exactly one instance of some piece of data throughout the whole system. If you think about it, that’s exactly what our miles-to-kilometers value is all about—we only need one instance of that number for the whole system, so we declare it as const (which, as we’ve already 100 | Chapter 3: Abstracting Ideas with Classes and Structs seen, is like a special case of static). A similar pattern crops up in lots of places in the .NET Framework class library. For example, on a computer running Windows, there is a specific directory containing certain OS system files (typically C:\Windows \system32). The class library provides a class called Environment which offers, among other things, a SystemDirectory property that returns that location, and since there’s only one such directory, this is a static property. Another common use for static is when we want to cache information that is expensive to calculate, or which is frequently reused by lots of different objects of the same type. To get a benefit when lots of objects use the common data, it needs to be available to all instances. Static Constructors We can even apply the static keyword to a constructor. This lets us write a special constructor that only runs once for the whole class. We could add the constructor in Example 3-43 to our Plane class to illustrate this. Example 3-43. Static constructor static Plane() { Console.WriteLine("Plane static constructor"); } With this code in place, you would see the message printed out by that constructor just once at the beginning of the program—static constructors run exactly once. In case you’re wondering, yes, static fields can be marked as readonly. And just as a normal readonly field can only be modified in a constructor, a static readonly field can only be modified in a static constructor. But when exactly do static constructors run? We know when regular members get initialized and when normal constructors run—that happens when we new up the ob- ject. Everything gets initialized to zero, and then our constructor(s) are called to do any other initialization that we need doing. But what about static initialization? The static constructor will run no later than the first time either of the following hap- pens: you create an instance of the class; you use any static member of the class. There are no guarantees about the exact moment the code will run—it’s possible you’ll see them running earlier than you would have expected for optimization reasons. Field initializers for static fields add some slight complication. (Remember, a field ini- tializer is an expression that provides a default value for a field, and which appears in the field declaration itself, rather than the constructor. Example 3-44 shows some ex- amples.) .NET initializes the statics in the order in which they are declared. So, if you Static Fields and Properties | 101 reference one static field from the initializer for another static field in the same class, you need to be careful, or you can get errors at runtime. Example 3-44 illustrates how this can go wrong. (Also, the .NET Framework is somewhat noncommittal about ex- actly when field initializers will run—in theory it has more freedom than with a static constructor, and could run them either later or earlier than you might expect, although in practice, it’s not something you’d normally need to worry about unless you’re writing multithreaded code that depends on the order in which static initialization occurs.) Example 3-44. Unwise ordering of static field initializers class Bar { public bool myField; } // Bad - null reference exception on construction class Foo { public static bool field2 = field1.myField; public static Bar field1 = new Bar(); } // OK - initialized in the right order class Foo { public static Bar field1 = new Bar(); public static bool field2 = field1.myField; } Summary We saw how to define classes from which we can create instances called objects, and that this can be useful when attempting to model real-world entities. We can also define value types, using the struct keyword, and the main difference is that when we assign variables or pass arguments, value types always copy the whole value, whereas ordinary classes (which are reference types) only copy a reference to the underlying object. We also saw a simpler kind of type: enum. This lets us define named sets of constant values, and is useful when we need a value representing a choice from a fixed set of options. So, now we know how to abstract basic ideas of information storage (through fields and simple properties) and manipulation (through functions and calculated proper- ties), using classes and objects. In the next chapter, we’re going to look at how we can extend these ideas further using a concept called polymorphism to model a hierarchy of related classes that can extend or refine some basic contract. 102 | Chapter 3: Abstracting Ideas with Classes and Structs CHAPTER 4 Extensibility and Polymorphism In the previous chapter, we saw how to define various types of classes and specify their members—fields, properties, and functions. In this chapter, we’re going to start by looking at this again in more detail, and try to understand what underlying concepts we’re implementing when we use these different coding patterns. We’ll then introduce a couple of new concepts—inheritance and polymorphism—and the language features that help us implement them. We’ve finished our ATC application, by the way. Having gotten a reputation for build- ing robust mission-critical software on time and to spec, we’ve now been retained by the fire department to produce a training and simulation system for them. Exam- ple 4-1 shows what we have so far. Example 4-1. Classes representing firefighters and fire trucks class Firefighter { public string Name { get; set; } public void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); } public void Drive(Firetruck truckToDrive, Point coordinates) { if (truckToDrive.Driver != this) { // We can't drive the truck if we're not the driver // But just silently failing is BADBAD // What we need is some kind of structured means // of telling the client about the failure // We'll get to that in Chapter 6 return; } truckToDrive.Drive(coordinates); } 103 } class Firetruck { public Firefighter Driver { get; set; } public void Drive(Point coordinates) { if (Driver == null) { // We can't drive if there's no driver return; } Console.WriteLine("Driving to {0}", coordinates); } } We have a model of the Firetruck, which uses a Firefighter as its Driver. The truck can be instructed to drive somewhere (if it has a driver), and you can tell a Firefighter to drive the truck somewhere (if he is the designated driver). You can think of this as modeling a relationship between a Firetruck and its Driver. That driver has to be a Firefighter. In object-oriented design, we call this relationship between classes an association. Association Through Composition and Aggregation An association is a kind of flexible, “arms length” relationship between two entities in the system. There are no particular constraints about the direction of the relationship: the firefighter can be associated with the truck, or the truck with the firefighter. Or both. Any particular firefighter may have associations with other types, and we can always assign another driver to the fire truck; there’s no exclusivity. For instance, we can do something like this: Firetruck truckOne = new Firetruck(); Firefighter joe = new Firefighter { Name = "Joe" }; Firefighter frank = new Firefighter { Name = "Frank" }; truckOne.Driver = joe; // Later... truckOne.Driver = frank; But what about the 30 foot retractable ladder that we happen to have on the fire truck; what kind of relationship exists between the ladder and the fire truck? Here’s our ladder class: class Ladder { public double Length { get; set; } } 104 | Chapter 4: Extensibility and Polymorphism This particular ladder is one of those powered, extensible rotating things that are built right into the truck. So let’s add a property to represent that (see Example 4-2). Example 4-2. Fire truck with integral ladder class Firetruck { public Firefighter Driver { get; set; } readonly Ladder ladder = new Ladder { Length = 30.0 }; public Ladder Ladder { get { return ladder; } } // ... } When we construct the Truck, it creates a 30-foot ladder for itself, with a read-only property to retrieve it. We call this “made of” association between classes composition. The ladder is a built- in part of the fire truck, but the fire truck can never be a part of the ladder, and the truck itself is responsible for the life cycle of its own ladder. What if we need to manage other equipment on the truck—a detachable coil of hose, for example: class Hose { } We could add a property to the Truck to get and set that (modeling a particular coil of hose being connected to the hose system on the truck): public Hose Hose { get; set; } This is another kind of composition relationship—one component of the Truck is a hose, and the truck certainly can’t be a part of the hose; but the containing object (the truck) no longer controls the creation and lifetime of its own piece of apparatus. Instead, we say that it aggregates the hose. Of course, there are no hard-and-fast rules about these terms and the code you write; they are just concepts which we use when we are designing systems. The definitions we’ve used come from the Unified Modeling Language (UML) 2.0, and we’re just mapping them to C# language features. Association Through Composition and Aggregation | 105 Nonetheless, it is useful to have a common conceptual language for describing our systems and the common characteristics of the code we use to implement them. Equally, when you are looking at someone else’s code (remembering that “someone else” includes “past you”) it is helpful to be able to translate what was written into these standard modeling concepts. So we have a software model of the Firetruck, which has a Ladder and a Hose and uses a Firefighter as its Driver. What about the fire chief? The fire chief is just another firefighter. He can drive a truck. He can put out fires. But he can do other stuff too. For instance, he can delegate responsibility for putting out a fire to another firefighter. The question we ask ourselves is this: is the FireChief a Firefighter with extra responsibilities? If the answer is yes, we are describing an is-a association (the FireChief is a Firefighter) which we can represent by an inheritance relationship. Inheritance and Polymorphism We’ll get into the nuances of the question in the preceding paragraph in a minute, but let’s assume for the time being that our answer to the question is yes (which, on face value, seems reasonable). Example 4-3 shows how we use inheritance in C#. Example 4-3. Inheritance in C# class FireChief : Firefighter { public void TellFirefighterToExtinguishFire (Firefighter colleague) { colleague.ExtinguishFire(); } } Notice that we use the colon in the class declaration to indicate that FireChief is a Firefighter. We then say that Firefighter is a base class of FireChief. Looking at the relationship from the other direction, we can also say that FireChief is a derived class of Firefighter. We’ve added the extra function that allows the chief to tell a firefighter to extinguish a fire—which encapsulates that extra responsibility. What we haven’t had to do is to duplicate all the functionality of the firefighter; that comes along anyway. We can now use the fire chief just as we would a firefighter, as shown in Example 4-4. Example 4-4. Using base class functionality inherited by a derived class Firetruck truckOne = new Firetruck(); FireChief bigChiefHarry = new FireChief { Name = "Harry" }; truckOne.Driver = bigChiefHarry; bigChiefHarry.Drive(truckOne, new Point(100,300)); 106 | Chapter 4: Extensibility and Polymorphism Firefighter joe = new Firefighter { Name = "Joe" }; bigChiefHarry.TellFirefighterToExtinguishFire(joe); Because bigChiefHarry is an object of type FireChief, and a FireChief is a Fire fighter, we can assign him to be the driver of a truck and tell him to drive it somewhere. But because he is a FireChief, we can also ask him to tell Joe to put out the fire when he gets there. Wherever we talk about a FireChief, we can treat the object as a Firefighter. This use of one type as though it were one of its bases is an example of polymorphism. Equally, we could phrase that the other way around: we can successfully substitute an instance of a more-derived class where we expect a base class. This is known as the Liskov Substitution Principle (LSP) after computer scientist Barbara Liskov, who ar- ticulated the idea in a paper she delivered in 1987. It is quite possible to derive one class from another in a way that means we can’t treat the derived class as its base type. The derived class could change the meaning or behavior of a function with the same signature as its base, or throw errors in situations where the base promised that everything would be fine—say, the base accepted parameters in the range 1–10, where the derived class accepts parameters in the range 2–5. This violates the LSP, which is a very poor design practice, but it is very easy to slip into, especially if the classes evolve independently over time. What happens if our client doesn’t know that Harry is a fire chief, though? What if we refer to the object via a reference typed to Firefighter instead? FireChief bigChiefHarry = new FireChief { Name = "Harry" }; // Another reference to Harry, but as a firefighter Firefighter stillHarry = bigChiefHarry; Firefighter joe = new Firefighter { Name = "Joe" }; stillHarry.TellFirefighterToExtinguishFire(joe); You know that stillHarry is referencing an object that is a FireChief, with that extra method on it. But the compiler produces a long, semicomprehensible error full of useful suggestions if you try to compile and execute this code: 'Firefighter' does not contain a definition for 'TellFirefighterToExtinguishFire' and no extension method 'TellFirefighterToExtinguishFire' accepting a first argument of type 'Firefighter' could be found (are you missing a using directive or an assembly reference?) Inheritance and Polymorphism | 107 The compiler is being rather tactful. It is assuming that you must’ve forgotten to include some external reference that’s got a suitable extension method definition to fix your problem. (We’ll be looking at that technique in a later chapter, by the way.) Unfortunately, the real reason for our bug is hidden in the error’s opening salvo: we’re trying to talk to a FireChief method through a variable that is strongly typed to be a Firefighter, and you can’t call on any members of the derived class through a reference typed to a base. So, if we can’t use a derived member from a reference to a base type, is there any way we can refine these classes so that Harry never puts out a fire, but always passes re- sponsibility to his Number One when he’s asked to do so, regardless of whether we happen to know that he’s a FireChief? After all, he knows that he’s the boss! To get started, we’ll have to make a few changes to the model to accommodate this idea of the chief’s Number One. In other words, we need to create an association be- tween the FireChief and his NumberOne. Remember that we typically implement this as a read/write property, which we can add to the FireChief: public Firefighter NumberOne { get; set; } And let’s change the main function so that it does what we want (see Example 4-5). Example 4-5. Using base class methods to keep the compiler happy // A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; // Firefighter harry is really a firechief, with joe as his NumberOne Firefighter harry = new FireChief { Name = "Harry", NumberOne = joe }; // Harry is just a firefighter, so he can extinguish fires // but we want him to get joe to do the work harry.ExtinguishFire(); But if we compile that, here’s the output we get: Harry is putting out the fire! That’s not what we want at all. What we want is a different implementation for that ExtinguishFire method if we’re actually a FireChief, rather than an ordinary Firefighter. 108 | Chapter 4: Extensibility and Polymorphism Replacing Methods in Derived Classes So the implementation for the ExtinguishFire method that we want on the FireChief looks like this: public void ExtinguishFire() { // Get our number one to put out the fire instead TellFirefighterToExtinguishFire(NumberOne); } What happens if we just add that function to our FireChief and compile and run? Well, it compiles, but when we run it, it still says: Harry is putting out the fire! It seems to have completely ignored our new function! Let’s go back and have a look at that compiler output again. You’ll see that although it built and ran, there’s a warning (you may have to rebuild to get it to appear again; Choose Rebuild Solution from the Build menu): 'FireChief.ExtinguishFire()' hides inherited member 'Firefighter.ExtinguishFire()'. Use the new keyword if hiding was intended. It is a good idea to leave all your compiler warnings on and work until you are both error and warning free. That way, when something crops up unexpectedly like this, you can spot it easily, rather than burying it in a pile of stuff you’re habitually ignoring. It is telling us that, rather than replacing the implementation on the base class, our method (with matching signature) is hiding it; and that if this is what we really meant to do, we should add the keyword new to the method. Hiding Base Members with new OK, let’s do that: public new void ExtinguishFire() { // Get our number one to put out the fire instead TellFirefighterToExtinguishFire(NumberOne); } We typically add the new modifier between the accessibility modifier and the return value. Replacing Methods in Derived Classes | 109 Compile and run again. You’ll notice that we’ve gotten rid of the warning, but the output hasn’t changed: Harry is putting out the fire! What’s going on? This method-hiding approach is actually letting a single object provide different im- plementations for the ExtinguishFire method. The implementation we get is based on the type of the variable we use, rather than the type of object to which the variable refers. You can see that happening if we use the code in Example 4-6 in our client. Example 4-6. Different reference type, different method // A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; // Firefighter harry is really a firechief, with joe as his NumberOne FireChief harry = new FireChief { Name = "Harry", NumberOne = joe }; Firefighter harryAsAFirefighter = harry; // Harry is just a firefighter, so he can extinguish fires // but as a firechief he gets joe to do the work harry.ExtinguishFire(); // While as a firefighter he does it himself harryAsAFirefighter.ExtinguishFire(); The output we get now looks like this: Joe is putting out the fire! Harry is putting out the fire! When we talk to our Harry object through a FireChief reference, he gets Joe to put out the fire. If we talk to the object through a Firefighter reference, he does it himself. Same object, but two completely different implementations. Why might we want to do that? Let’s say we had multiple fire chiefs on a job, but it is our policy that a chief acting as another chief’s Number One is not allowed to delegate the job again. Our code models exactly this behavior, as shown in Example 4-7. Of course, whether that’s desirable behavior is another matter entirely— we’ve ended up with such radically different approaches to putting out a fire that it might be better to separate them back out into functions with different names. When you go through a refactoring process such as this, it is a good idea to check that you’re still happy with the semantic implications of your code. Ideally, you want to end up with a neat design, but a superficially neat design that makes no sense is not helpful. 110 | Chapter 4: Extensibility and Polymorphism Example 4-7. Making twisted use of method hiding // A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; // FireChief harry has joe as his NumberOne FireChief harry = new FireChief { Name = "Harry", NumberOne = joe }; FireChief tom = new FireChief { Name = "Tom", NumberOne = harry }; // Harry is just a firefighter, so he can extinguish fires // but as a firechief he gets joe to do the work harry.ExtinguishFire(); // But if Tom is asked to extinguish a fire, he asks Harry to do it // Our policy dictates that Harry has to do it himself, not delegate to // Joe this time. tom.ExtinguishFire(); Harry delegates to Joe when he is asked to do it himself, because we are calling through a reference to a FireChief. Tom is also a FireChief, and we are calling through a reference to him as a FireChief, so he delegates to Harry; but when Harry is asked to do it in his role as a Firefighter (remember, the NumberOne property is a reference to a Firefighter), he does it himself, because we are now calling the method through that reference typed to Firefighter. So our output looks like this: Joe is putting out the fire! Harry is putting out the fire! That’s all very well, but we don’t actually want that restriction—the fire chief should be allowed to pass the work off to his subordinate as often as he likes, regardless of who he asked to do it. There’s one big caveat regarding everything we’ve just shown about method hiding: I can’t think of the last time I used this feature in a real application, but I see the warning from time to time and it usually alerts me to a mistake in my code. We’ve wanted to illustrate how method hiding works, but we discour- age you from using it. The main reason to avoid method hiding with new is that it tends to surprise your clients, and that, as we’ve established, is not a good thing. (Would you really expect behavior to change be- cause the type of the variable, not the underlying object, changes?) While method hiding is absolutely necessary for some corner cases, we usually treat this warning as an error, and think very carefully about what we’re doing if it comes up. 9 times out of 10, we’ve got an inad- vertent clash of names. Replacing Methods in Derived Classes | 111 Replacing Methods with virtual and override What we actually want to do is to change the implementation based on the type of the object itself, not the variable we’re using to get at it. To do that we need to replace or override the default implementation in our base class with the one in our derived class. A quick glance at the C# spec shows us that there is a keyword to let us do just that: override. Let’s switch to the override modifier on the FireChief implementation of the ExtinguishFire() method: public override void ExtinguishFire() { // Get our number one to put out the fire instead TellFirefighterToExtinguishFire(NumberOne); } Notice that we removed the new modifier and replaced it with override instead. But if you compile, you’ll see that we’re not quite done (i.e., we get a compiler error): 'FireChief.ExtinguishFire()': cannot override inherited member 'Firefighter.ExtinguishFire()' because it is not marked virtual, abstract, or override We’re not allowed to override the method with our own implementation because our base class has to say we’re allowed to do so. Fortunately, we wrote the base class, so we can do that (as the compiler error suggests) by marking the method in the base with the virtual modifier: class Firefighter { public virtual void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); } // ... } Why do we have this base-classes-opt-in system? Why is everything not virtual by de- fault (like, say, Java)? Arguments continue on this very issue, but the designers of C# chose to go with the nonvirtual-by-default option. There are a couple of reasons for this: one has to do with implicit contracts, and another is related to versioning. There is also (potentially) a small performance overhead for virtual function dispatch, but this is negligible in most real-world scenarios. As always, test before optimizing for this! 112 | Chapter 4: Extensibility and Polymorphism We already saw how our public API is effectively a contract with our clients. With virtual functions, though, we are defining not only a contract for the caller, as usual, but also a contract for anyone who might choose to override that method. That requires more documentation, and a greater degree of control over how you implement the method. By declaring a method as virtual, the base class gives derived classes permission to replace whole pieces of its own innards. That’s a very powerful but very dangerous technique, rather like organ transplant surgery on an animal you’ve never seen before. Even a trained surgeon might balk at replacing the kidneys of a dromedary armed with nothing more than developer-quality documentation about the process. For example, some method in your base class calls its MethodA, then its MethodB, to do some work. You then (perhaps unknowingly) rely on that ordering when you provide overrides for MethodA and MethodB. If a future version of the base class changes that ordering, you will break. Let’s go back to our example to look at that in more detail, because it is really important. First, let’s change the implementation of Firefighter.ExtinguishFire so that it makes use of a couple of helper methods: TurnOnHose and TrainHoseOnFire (see Example 4-8). Example 4-8. Virtual methods and method ordering class Firefighter { // This calls TrainHoseOnFire and TurnOnHose as part of the // process for extinguishing the fire public virtual void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); TrainHoseOnFire(); TurnOnHose(); } private void TurnOnHose() { Console.WriteLine("The fire is going out."); } private void TrainHoseOnFire() { Console.WriteLine("Training the hose on the fire."); } // ... } Replacing Methods in Derived Classes | 113 Let’s also simplify our Main function so that we can see what is going on, as shown in Example 4-9. Example 4-9. Calling a virtual method static void Main(string[] args) { // A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; joe.ExtinguishFire(); Console.ReadKey(); } If we compile and run, we’ll see the following output: Joe is putting out the fire! Training the hose on the fire. The fire is going out. All is well so far, but what happens if we add a trainee firefighter into the mix? The trainee is extremely fastidious and follows his instructor’s guidelines to the letter. We’re going to make a class for him and override the TurnOnHose and TrainHoseOnFire methods so that the work is done in the trainee’s own particular idiom. Hang on a moment, though! Our helper methods are private members. We can’t get at them, except from other members of our Firefighter class itself. Before we can do anything, we need to make them accessible to derived classes. Inheritance and Protection In the preceding chapter, we mentioned that there were two additional accessibility modifiers that we would deal with later: protected and protected internal. Well, this is where they come into their own. They make members accessible to derived classes. If you want a member to be available either to derived classes or to other classes in your own assembly, you mark that member protected internal. It will be visible to other classes in the library, or to clients that derive classes from your base, but inaccessible to other clients who just reference your assembly. If, on the other hand, you want your class to make certain methods available only to derived classes, you just mark those methods protected. In terms of code out there in the wild, this is the most common usage, but it is not necessarily the best one! 114 | Chapter 4: Extensibility and Polymorphism Both protected internal and internal are much underused access modifiers. They are a very convenient way of hiding away library im- plementation details from your consumers, and reducing the amount of documentation and surface-area testing you need. I suspect that they are unpopular (as with most “hidden by default” or “secure by default” schemes) because they can sometimes get in your way. There are a fair number of implementation details of classes in the .NET Framework that are internal (or private) that people would very much like to access, for example. A common reason for taking something useful and applying the inter nal modifier is that it was not possible to fully document (or understand the full implications of) the “hook” this would provide into the frame- work. And rather than open up potential security or reliability problems, they are marked internal until a later date: perhaps much, much later, tending toward never. Although there is an intention to revisit these things, real-world pressures mean that they often remain unchanged. This is another example of the “lock down by default” strategy which helps improve software quality. That doesn’t make it any less irritating when you can’t get at the inner workings, though! So we’ll mark those methods in the base class virtual and protected, as shown in Example 4-10. Example 4-10. Opening methods up to derived classes protected virtual void TurnOnHose() { Console.WriteLine("The fire is going out."); } protected virtual void TrainHoseOnFire() { Console.WriteLine("Training the hose on the fire."); } We can now create our TraineeFirefighter class (see Example 4-11). Example 4-11. Overriding the newly accessible methods class TraineeFirefighter : Firefighter { private bool hoseTrainedOnFire; protected override void TurnOnHose() { if (hoseTrainedOnFire) { Console.WriteLine("The fire is going out."); } Inheritance and Protection | 115 else { Console.WriteLine("There's water going everywhere!"); } } protected override void TrainHoseOnFire() { hoseTrainedOnFire = true; Console.WriteLine("Training the hose on the fire."); } } As you can see, the trainee is derived from Firefighter. We added an extra Boolean field to keep track of whether the trainee has actually trained the hose on the fire, and then provided our own implementations of TrainHoseOnFire and TurnOnHose that make use of that extra field. This is intended to model the detailed but slightly peculiar and occasionally erratic way in which the trainee follows the instructions for these opera- tions in his copy of How to Be a Firefighter, rather than allowing common sense to prevail. We also need a quick update to our main function to use our trainee. Let’s add the following code at the end: // A reference to Bill, the trainee Firefighter bill = new TraineeFirefighter { Name = "Bill" }; bill.ExtinguishFire(); If we compile and run, we see the following output: Joe is putting out the fire! Training the hose on the fire. The fire is going out. Bill is putting out the fire! Training the hose on the fire. The fire is going out. Well done, Bill; all that training came in handy, exactly as we’d expect. Although it works, you’ll notice that we’ve duplicated some code from our base class into our derived class—the bit that actually does the work in each of those methods. It would be better if we could just call on our base class implementation to do the job for us. As you’d expect, C# has this in hand, with the base keyword. Calling Base Class Methods If we ever want to call on the implementation of a member in our base class (bypassing any of our own overrides), we can do so through the special base name: base.CallOnTheBase(); 116 | Chapter 4: Extensibility and Polymorphism Using that, we can reimplement our TraineeFirefighter and remove that duplicate code, as shown in Example 4-12. Example 4-12. Avoiding duplication by calling the base class class TraineeFirefighter : Firefighter { private bool hoseTrainedOnFire; protected override void TurnOnHose() { if (hoseTrainedOnFire) { // Call on the base implementation base.TurnOnHose(); } else { Console.WriteLine("There's water going everywhere!"); } } protected override void TrainHoseOnFire() { hoseTrainedOnFire = true; base.TrainHoseOnFire(); } } So, what happens if in a later version we change the implementation of the Extinguish Fire method on the base class? Maybe we found an optimization that means it is faster to implement it like this: public virtual void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); // We've swapped these around TurnOnHose(); TrainHoseOnFire(); } Let’s imagine that this Firefighter class is being implemented by one of our colleagues. She tested the new implementation against her Firefighter unit test suite, exactly as required, and everything passed just fine—fires were extinguished. Then she handed it over to us to use (with our new TraineeFirefighter class that we’re working on). If we compile and run, we get the following output: Joe is putting out the fire! Training the hose on the fire. The fire is going out. Bill is putting out the fire! There's water going everywhere! Training the hose on the fire. Calling Base Class Methods | 117 So the Firefighter code works fine, just as our colleague promised; but our Trainee Firefighter has made a bit of a mess. This is a shame, as he has not done anything different—we didn’t change a single line of our TraineeFirefighter code that was working just a moment earlier. The problem is that, while our documentation for ExtinguishFire told us that it would call both of those virtual methods it didn’t promise to do so in any particular order. And there was no documentation at all on our protected virtual methods to tell us how we should override them, or whether there were any particular constraints or invariants we should maintain. This is a very common combination of problems when designing an inheritance hierarchy—poor documentation on the base class, and in- sufficiently defensive implementations in a derived class. Creating a class hierarchy is not an easy thing to do. And this is when we’re only making selected methods virtual—imagine the chaos if all methods were virtual by default! In the next chapter, we’re going to look at some alternative ways to vary behavior that are more easily documented and potentially more robust than deriving from a base class. That’s not to say that you shouldn’t make use of such a powerful concept as polymorphism; it is just that you should take care when you do so. Let’s just recap the implications of all that, as it is a bit complicated. Back in Chapter 3, when we designed our first class, we talked about its public contract, and how that encapsulated the implementation details which allowed us to use it as though it was a black box. With the addition of public and protected virtual members, we’re opening that black box and creating a second contract: for people who derive their own classes, which, as we just saw, is a whole lot more complex. The designers of C# decided that should be an opt-in complexity: unless we specify that a member is virtual we don’t have to worry about it. Along the same lines, they’ve also provided a way to ensure that we don’t have to worry about anyone deriving from us at all. Thus Far and No Farther: sealed Having got through all of that, you’re probably rightly concerned that, simple though it is in theory, the practical implications of inheritance are actually rather complex and require a lot of documentation, testing, and imagining how people might use and abuse your virtual methods. And we have to do that for every class down the hierarchy. 118 | Chapter 4: Extensibility and Polymorphism When we designed our FireChief, we happily provided an override for the Extinguish Fire method, without giving a thought for the fact that someone else might override that method in his own derived class. In fact, we didn’t even consider the possibility that anyone might derive from FireChief at all. No documentation, nothing. Now there are several members on our own base class that could be overridden by a class that derives from FireChief. Does that have any implications for our own docu- mentation or testing? Can we even tell? And how could we have guessed that was going to happen when we built our FireChief class, since there was only one virtual member on the base at that time? This looks like it has the potential to become a rich future source of bugs and security holes. Fortunately, we can eliminate this problem at a stroke by saying that we didn’t design our FireChief to be derived from, and stopping anyone from doing so. We do that by marking the FireChief class sealed. Let’s see how that looks: sealed class FireChief : Firefighter { // ... } We apply the sealed modifier before the class keyword and after any accessibility modifiers if they are present. So, what happens if we try to derive a new class from FireChief now? class MasterChief : FireChief { } Compile it, and you’ll see the following error: 'MasterChief': cannot derive from sealed type 'FireChief' That’s put a stop to that. Let’s delete our MasterChief so that everything builds again. Not only can sealing classes be very useful (and defensive), but if you decide later that you want to unseal the class and allow people to derive their own types, it doesn’t (normally) break binary compatibility for the type. Sealing a previously unsealed class, however, does break compatibility. We now have three different types of firefighter. Let’s remind ourselves how they are related (see Figure 4-1). Thus Far and No Farther: sealed | 119 Figure 4-1. Three types of firefighter Nonvirtual by Default, but Not sealed? Why, you may ask, if we are nonvirtual by default aren’t we also sealed by default, with an “unseal” keyword? Notice, for instance, that we’ve been talking about classes so far—value types (struct) are sealed (with no opt-out), so you can’t derive from them. There’s no performance hit to marking a class sealed. There are potential security ad- vantages to marking a class sealed (no one can sneakily exploit polymorphism to insert code where you weren’t expecting it). So why not make them all sealed? It is certainly much less problematic to present an unsealed class than it is to present a virtual method; if there are no virtual methods, all you can do is to bolt extra bits on, which do no harm to anyone. It also conforms to the expectations of a generation of C++ and Java developers in this regard. Plenty of people argue that we should have both unsealed-by-default and virtual-by- default, and they certainly have a point, particularly with regard to convenience; but the designers of C# took a different view. No doubt, the debate will continue. Those three types of firefighter basically differ in the strategy that they use for putting out fires. There’s a base class that provides a default implementation, and a couple of classes that override the virtual methods to do things differently. Let’s say we wanted to support lots of different types of firefighter, all of whom were expected to have a different approach to fighting fire, from the trainee, to the chief, to Gulliver (who has his own idiosyncratic way of putting out a fire in Lilliput). We still want the handy Name property and the Drive method, and we still want anyone to be able to call an ExtinguishFire method. 120 | Chapter 4: Extensibility and Polymorphism Noticing that our FireChief, for example, doesn’t make use of the base implementation at all; we don’t want to provide a standard for that method. We’ll just let all imple- menters decide for themselves how it is going to work. We’re shooting for something that looks like Figure 4-2. Figure 4-2. Abstract base classes Requiring Overrides with abstract An abstract base class is intended to provide the “scaffolding” for a hierarchy of related classes, but it is not intended to be instantiated itself, because it isn’t “finished.” It requires that classes derived from it add in some missing bits. Let’s turn our current firefighter into an abstract base for the others to use, and see how that works. First, we can add the abstract modifier to the class itself, and see what happens: abstract class Firefighter { // ... } As usual, we add the modifier before the class keyword (and after any accessibility modifiers, if present). If we build, we now get a compiler error: Cannot create an instance of the abstract class or interface 'Firefighter' Requiring Overrides with abstract | 121 That’s because we’re trying to create an instance of the Firefighter in our main function: Firefighter joe = new Firefighter { Name = "Joe" }; This is no longer allowed, because the Firefighter class is now abstract. OK, we’ll comment that out temporarily while we carry on refactoring. We want it to continue to build as we go so that we can see if we’ve introduced any other errors: //Firefighter joe = new Firefighter { Name = "Joe" }; //joe.ExtinguishFire; Build and run, and we get the output we expect—Bill is still spraying the water around: Bill is putting out the fire! There's water going everywhere! Training the hose on the fire. One other thing: if we’re creating an abstract base class, we usually name it something such as FooBase to distinguish it from a regular class. This is by no means a hard-and- fast rule, but it is pretty common. So let’s rename Firefighter to FirefighterBase, and make sure we change it where it is referenced elsewhere—on the Firetruck, FireChief, and TraineeFirefighter classes. The easiest way to do that is to use the automatic rename refactoring in the IDE. Just type over the old name in the declaration, click on the Smart Tag that appears, and choose Rename Firefighter to FirefighterBase from the menu. You could do it by hand if you wanted, though. The whole purpose of this was to get rid of the default implementation we have for putting out fires, so let’s turn Firefighterbase.ExtinguishFire into an abstract method. Just like the modifier for the class, we use the abstract keyword, but this time we also remove the method body and add a semicolon at the end of the declaration: abstract class FirefighterBase { public abstract void ExtinguishFire(); } If you try building again now, you can see that we have a new compiler error: 'TraineeFirefighter' does not implement inherited abstract member 'FirefighterBase.ExtinguishFire()' Remember, we are required to override an abstract method; our class isn’t finished until we do so (unlike a virtual method, where we are invited to override it, but it will fall back on the base if we don’t). While our FireChief does override the method, our TraineeFirefighter doesn’t. So we need to add a suitable implementation: class TraineeFirefighter : FirefighterBase { // Override the abstract method public override void ExtinguishFire() 122 | Chapter 4: Extensibility and Polymorphism { // What are we going to put here? } // ... } But what are we going to put into that ExtinguishFire override? Before, we depended on our base class for the implementation, but our base is now abstract, so we don’t have one available anymore! That’s because we’ve forgotten about our regular Firefighter. Let’s add a class for him back into the hierarchy: class Firefighter : FirefighterBase { public override void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); TurnOnHose(); TrainHoseOnFire(); } } Notice we’ve given him the “standard firefighter” implementation for ExtinguishFire. If we take one more look at the base class, we can see that we still have those two virtual implementation helpers. While everything builds correctly at the moment, they don’t really belong there; they are really a part of the Firefighter implementation, so let’s move them in there. We end up with the code in Example 4-13. Example 4-13. Refactored base classes abstract class FirefighterBase { public abstract void ExtinguishFire(); public string Name { get; set; } public void Drive(Firetruck truckToDrive, Point coordinates) { if (truckToDrive.Driver != this) { // We can't drive the truck if we're not the driver return; } truckToDrive.Drive(coordinates); } } class Firefighter : FirefighterBase { public override void ExtinguishFire() { Console.WriteLine("{0} is putting out the fire!", Name); TrainHoseOnFire(); Requiring Overrides with abstract | 123 TurnOnHose(); } protected virtual void TurnOnHose() { Console.WriteLine("The fire is going out."); } protected virtual void TrainHoseOnFire() { Console.WriteLine("Training the hose on the fire."); } } But we’re still not quite done! If you build this you’ll see another compiler error: 'TraineeFirefighter.TurnOnHose()': no suitable method found to override 'TraineeFirefighter.TrainHoseOnFire()': no suitable method found to override Our trainee firefighter really is a kind of firefighter, and depends on those two virtual functions we just moved. The error message is telling us that we can’t override a method that isn’t actually present in the base. We need to change its base class from FirefighterBase to Firefighter. This has the advantage that we can also get rid of its duplicate override of the ExtingushFire method (see Example 4-14). Example 4-14. Using the newly refactored base classes class TraineeFirefighter : Firefighter { protected override void TurnOnHose() { if (hoseTrainedOnFire) { Console.WriteLine("The fire is going out."); } else { Console.WriteLine("There's water going everywhere!"); } } private bool hoseTrainedOnFire; protected override void TrainHoseOnFire() { hoseTrainedOnFire = true; Console.WriteLine("Training the hose on the fire."); } } 124 | Chapter 4: Extensibility and Polymorphism We also need to uncomment our two lines about Joe in the Main function—everything should work again: Firefighter joe = new Firefighter { Name = "Joe" }; joe.ExtinguishFire(); We can build and run to check that. We get the expected output: Joe is putting out the fire! Training the hose on the fire. The fire is going out. Bill is putting out the fire! There's water going everywhere! Training the hose on the fire. Let’s remind ourselves of the current class hierarchy (see Figure 4-2). Our FireChief is no longer an “ordinary” Firefighter, with an override for putting out fires, but he does take advantage of our common scaffolding for “firefighters in general” that we modeled as an abstract base class called FirefighterBase. Our Firefighter also takes advantage of that same scaffolding, but our TraineeFirefighter really is a Firefighter—just with its own idiosyncratic way of doing some of the internal methods that Firefighter uses to get the job done. Back to the requirements for our fire department application: let’s say we want to keep track of who is actually in the fire station at any particular time, just in case there is a fire on the premises and we can take a roll call (health and safety is very important, especially in a fire station). There are two types of folks in the fire station: the firefighters and the administrators. Example 4-15 shows our new Administrator class. Example 4-15. A class representing administrative staff class Administrator { public string Title { get; set; } public string Forename { get; set; } public string Surname { get; set; } public string Name { get { StringBuilder name = new StringBuilder(); AppendWithSpace(name, Title); AppendWithSpace(name, Forename); AppendWithSpace(name, Surname); return name.ToString(); } } void AppendWithSpace(StringBuilder builder, string stringToAppend) { // Don't do anything if the string is empty Requiring Overrides with abstract | 125 if (string.IsNullOrEmpty(stringToAppend)) { return; } // Add a space if we've got any text already if (builder.Length > 0) { builder.Append(" "); } builder.Append(stringToAppend); } } If you look at our Firefighter class, it had a single string property for a Name. With the Administrator, you can independently get and set the Title, Forename, and Surname. We then provided a special read-only property that returns a single formatted string for the whole Name. It uses a framework class called StringBuilder to assemble the name from the individual components as efficiently as possible. AppendWithSpace is a utility function that does the actual work of concatenating the substrings. It works out whether it needs to append anything at all using a static method on string that checks whether it is null or empty, called IsNullOrEmpty; finally, it adds an extra space to separate the individual words. To do the roll call we want to write some code such as that in Example 4-16. Example 4-16. Using the Administrator class static void Main(string[] args) { FireStation station = new FireStation(); // A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; // A reference to Bill, the trainee FirefighterBase bill = new TraineeFirefighter { Name = "Bill" }; // Harry is back FireChief bigChiefHarry = new FireChief { Name = "Harry"}; // And here's our administrator - Arthur Administrator arthur = new Administrator { Title = "Mr", Forename = "Arthur", Surname = "Askey" }; station.ClockIn(joe); station.ClockIn(bill); station.ClockIn(bigChiefHarry); station.ClockIn(arthur); 126 | Chapter 4: Extensibility and Polymorphism station.RollCall(); Console.ReadKey(); } When you are designing a class framework it can often be a good idea to write some example client code. You can then ensure that your design is a good abstract model while supporting clean, simple code at point- of-use. Clearly, we’re going to need a FireStation class that is going to let our administrators and firefighters ClockIn (registering their presence in the station), and where we can do a RollCall (displaying their names). But what type is that ClockIn function going to take, given that we haven’t specified any common base class that they share? All Types Are Derived from Object .NET comes to our rescue again. It turns out that every type in the system is derived from Object. Every one—value types (struct) and reference types (class) alike, even the built-in types such as Int32. It is easy to see how that would work for a class declaration in C#. If you don’t specify a particular base class, you get Object by default. But what about a struct, or enum, or the built-in types; what happens if we try to talk to them through their Object “base class”? Boxing and Unboxing Value Types Let’s give it a try. This code snippet will compile and work quite happily: // Int variable int myIntVariable = 1; object myObject = myIntVariable; What happens under the covers is that the runtime allocates a new object and puts a copy of the value inside it. This is called boxing, and, as you might expect given that it involves allocating objects and copying values, it is relatively expensive when compared to a straightforward assignment. You can also convert back the other way: // Int variable int myIntVariable = 1; object myObject = myIntVariable; int anotherIntVariable = (int)myObject; All Types Are Derived from Object | 127 Notice how we use the type name in parentheses to perform the conversion back to an int. In general, this sort of conversion from one type to another is known as a “cast,” and will work for classes too (although we’ll see a more explicit way of doing that later in this chapter). The runtime looks at that box object for us and checks that it contains a value of the correct type. If so, it will copy the value back out of the box and into the new variable. What if it isn’t of the correct type? The runtime will throw an Invalid CastException. You can find out more about exceptions in Chapter 6. That process is known as unboxing, and is also quite expensive (although not as ex- pensive as boxing, as it doesn’t need to allocate the object). Although these performance costs are individually fairly small, if you are processing large numbers of value types in a way that requires them to be repeatedly boxed and unboxed the costs can add up quite rapidly; so you should be aware of boxing and unboxing when you are profiling your application. So the only common base of both Firefighter and Administrator is Object at the mo- ment (remember, everything is ultimately derived from Object). That seems a bit low level, but it is all we have to go on for now, so we’ll make do. Example 4-17 shows our first pass at a FireStation. Example 4-17. FireStation class class FireStation { List