深入理解.NET(第2版.英文影印版)(.NET领域的经典著作)(china-pub 全国首发）

.　17　custom attributes
　18　nullable value types
part v　clr facilities
　19　exceptions
　20　automatic memory management(garbage collection)
　21　clr hosting and appdomains
　22　assembly loading and reflection
　23　performing asynchronous operations
　24　thread synchronization
index

.　　■ Simplified programming model The CLR seeks to greatly simplify the plumbing and arcane constructs required by Win32 and COM. Specifically, the CLR now frees the developer from having to understand any of the following concepts: the registry, globally unique identifiers (GUIDs), IUnknown, AddRef, Release, HRESULTs, and so on. The CLR doesn’t just abstract these concepts away from the developer; these concepts simply don’t exist in any form in the CLR. Of course, if you want to write a .NET Framework application that interoperates with existing, non-.NET code, you must still be aware of these concepts.
　　■ Run once, run always All Windows developers are familiar with “DLL hell” versioning problems. This situation occurs when components being installed for a new application overwrite components of an old application, causing the old application to exhibit strange behavior or stop functioning altogether. The architecture of the .NET Framework now isolates application components so that an application always loads the components that it was built and tested with. If the application runs after installation, the application should always run.
　　■ Simplified deployment Today, Windows applications are incredibly difficult to set up and deploy. Several files, registry settings, and shortcuts usually need to be created. In addition, completely uninstalling an application is nearly impossible. With Windows 2000, Microsoft introduced a new installation engine that helps with all of these issues, but it’s still possible that a company authoring a Microsoft installer package might fail to do everything correctly. The .NET Framework seeks to banish these issues into history. The .NET Framework components are not referenced by the registry. In fact, installing most .NET Framework applications requires no more than copying the files to a directory and adding a shortcut to the Start menu, desktop, or Quick Launch toolbar. Uninstalling
　　the application is as simple as deleting the files.
　　■ Wide platform reach When compiling source code for the .NET Framework, the compilers produce common intermediate language (CIL) instead of the more traditional CPU instructions. At run time, the CLR translates the CIL into native CPU instructions. Because the translation to native CPU instructions is done at run time, the translation is done for the host CPU. This means that you can deploy your .NET Framework application on any machine that has an ECMA-compliant version of the CLR and FCL running on it. These machines can be x86, x64, IA64, and so on. Users will immediately appreciate the value of this broad execution if they ever change their computing hardware or
　　operating system. ..
　　■ Programming language integration COM allows different programming languages to interoperate with one another. The .NET Framework allows languages to be integrated with one another so that you can use types of another language as if they were your own. For example, the CLR makes it possible to create a class in C++ that derives from a class implemented in Visual Basic. The CLR allows this because it defines and provides a Common Type System (CTS) that all programming languages that target the CLR must use. The Common Language Specification (CLS) describes what compiler implementers must do in order for their languages to integrate well with other languages. Microsoft is itself providing several compilers that produce code that targets the runtime: C++/CLI, C#, Visual Basic .NET, and JScript. In addition, companies other than Microsoft and academic institutions are producing compilers for other languages that also target the CLR.
　　■ Simplified code reuse Using the mechanisms described earlier, you can create your own classes that offer services to third-party applications. This makes it extremely simple to reuse code and also creates a large market for component vendors.
　　■ Automatic memory management (garbage collection) Programming requires great skill and discipline, especially when it comes to managing the use of resources such as files, memory, screen space, network connections, database resources, and so on. One of the most common bugs is neglecting to free one of these resources, ultimately causing the application to perform improperly at some unpredictable time. The CLR automatically tracks resource usage, guaranteeing that your application will never leak resources. In fact, there is no way to explicitly “free” memory. In Chapter 20, “Automatic Memory Management (Garbage Collection),” I explain exactly how garbage collection works.
　　■ Type-safe verification The CLR can verify that all of your code is type-safe. Type safety ensures that allocated objects are always accessed in compatible ways. Hence, if a method input parameter is declared as accepting a 4-byte value, the CLR will detect and trap attempts to access the parameter as an 8-byte value. Similarly, if an object occupies 10 bytes in memory, the application can’t coerce the object into a form that will allow more than 10 bytes to be read. Type safety also means that execution flow will transfer only to well-known locations (that is, method entry points). There is no way to construct an arbitrary reference to a memory location and cause code at that location to start executing. Together, these measures ensure type safety, which eliminates many common programming errors and classic security attacks (for example, exploiting buffer overruns).
　　■ Rich debugging support Because the CLR is used for many programming languages, it is now much easier to implement portions of your application by using the language best suited to a particular task. The CLR fully supports debugging applications that cross language boundaries.
　　■ Consistent method failure paradigm One of the most annoying aspects of Windows programming is the inconsistent style that functions use to report failures. Some functions return Win32 status codes, some functions return HRESULTs, and some functions throw exceptions. In the CLR, all failures are reported via exceptions—period. Exceptions allow the developer to isolate the failure recovery code from the code required to get the work done. This separation greatly simplifies writing, reading, and maintaining code. In addition, exceptions work across module and programming language boundaries. And, unlike status codes and HRESULTs, exceptions can’t be ignored. The CLR also provides built-in stack-walking facilities, making it much easier to locate any bugs and failures.
　　■ Security Traditional operating system security provides isolation and access control based on user accounts. This model has proven useful but at its core assumes that all code is equally trustworthy. This assumption was justified when all code was installed from physical media (for example, CD-ROM) or trusted corporate servers. But with the increasing reliance on mobile code such as Web scripts, applications downloaded over the Internet, and e-mail attachments, we need ways to control the behavior of applications in a more code-centric manner. Code access security provides a means to do this.
　　■ Interoperability Microsoft realizes that developers already have an enormous amount of existing code and components. Rewriting all of this code to take full advantage of the .NET Framework platform would be a huge undertaking and would prevent the speedy adoption of this platform. So the .NET Framework fully supports the ability for developers to access their existing COM components as well as call Win32 functions in existing DLLs. Users won’t directly appreciate the CLR and its capabilities, but they will certainly notice the quality and features of applications that utilize the CLR. In addition, users and your company’s
　　bottom line will appreciate how the CLR allows applications to be developed and deployed more rapidly and with less administration than Windows has ever allowed in the past.
　　The Development Environment: Microsoft Visual Studio
　　Visual Studio is Microsoft’s development environment. Microsoft has been working on it for many years and has incorporated a lot of .NET Framework–specific features into it. Like any good development environment, Visual Studio includes a project manager; a source code editor; UI designers; lots of wizards, compilers, linkers, tools, and utilities; documentation; and debuggers. It supports building applications for both the 32-bit and 64-bit Windows platforms as well as for the .NET Framework platform. Another important improvement is that there is now just one integrated development environment for all programming languages and application types.
　　Microsoft also provides a .NET Framework SDK. This free SDK includes all of the language compilers, a bunch of tools, and a lot of documentation. Using this SDK, you can develop applications for the .NET Framework without using Visual Studio. You’ll just have to use your own editor and project management system. You also don’t get drag-and-drop Web Forms and Windows Forms building. I use Visual Studio regularly and will refer to it throughout this book.However, this book is mostly about .NET Framework and C# programming in general, so Visual Studio isn’t required to learn, use, and understand the concepts I present in each chapter.
　　The Goal of This Book
　　The purpose of this book is to explain how to develop applications and reusable classes for the .NET Framework. Specifically, this means that I intend to explain how the CLR works and the facilities it offers. I’ll also discuss various parts of the FCL. No book could fully explain the FCL—it contains literally thousands of types, and this number is growing at an alarming rate. So, here I’m concentrating on the core types that every developer needs to be aware of. And while this book isn’t specifically about Windows Forms, XML Web services, Web Forms, and so on, the technologies presented in the book are applicable to all of these application types.
　　With this book, I’m not attempting to teach you any particular programming language, although I use the C# programming in order to demonstrate features of the CLR and to access types in the FCL. I’m sure that you will learn a lot about C# as you go through this book, but it is not a goal of this book to teach C#. Furthermore, I assume that you are already familiar with object-oriented programming concepts such as data abstraction, inheritance, and polymorphism. A good understanding of these concepts is critical because the CLR offers an object-oriented programming model, and all of its features are exposed using this paradigm. If you’re not familiar with these concepts, I strongly suggest that you first find a book that teaches these concepts.
　　Sample Code and System Requirements
　　The samples presented in this book can be downloaded fromhttp://Wintellect.com. To build and run the samples, you’ll need the .NET Framework 2.0 (and a version of Windows that supports it) and the .NET Framework SDK.
　　This Book Has No Mistakes
　　This section’s title clearly states what I want to say. But we all know that it is a flat-out lie. My reviewers, editors, and I have worked hard to bring you the most accurate, up-to-date, in-depth, easy-to-read, painless-to-understand, bug-free information. Even with the fantastic team assembled, things inevitably slip through the cracks. If you find any mistakes in this book (especially bugs), I would greatly appreciate it if you would send the mistakes to me at JeffreyR@Wintellect.com.
　　Acknowledgments
　　I couldn’t have written this book without the help and technical assistance of many people. In particular, I’d like to thank the following people:
　　■ My family The amount of time and effort than goes into writing a book is hard to measure. All I know is that I could not have produced this book without the support of Kristin (my wife) and Aidan (my son). There were many times when we wanted to spend time together but were unable to due to book obligations. Now that the book project is completed, I really look forward to adventures we will all share together.
　　■ My technical reviewers and editors For this book revision, I truly had some fantastic people helping me. Christophe Nasarre has done just a phenomenal job of verifying my work and making sure that I’d said everything the best way it could possibly be said. He has truly had a significant impact on the quality of this book. Also, I’d like to extend a special thanks to Jamie Haddock. Jamie read the first edition of my book and e-mailed me numerous suggestions for ways to improve it. I saved all of these and then asked him to be part of the formal review process while writing the second edition of this book. Jamie’s contribution is also quite significant. I’d also like to thank Stan Lippman and Clemens Szyperski for their review and the lively discussions we had. Finally, I’d like to thank Paul Mehner for his feedback.
　　■ Members of the Microsoft Press editorial team The Microsoft Press people that I had the most contact with are Devon Musgrave and Joel Rosenthal. Both of them were an extreme pleasure to work with and made sure that things ran smoothly and did their best to make my words read good (except for this sentence ?). Of course, I’d also thank Ben Ryan, my acquisitions editor, for ushering the contract through. Finally, I’d also like to thank other Microsoft Press people who had a hand in this project, including Kerri Devault, Elizabeth Hansford, Dan Latimer, Patricia Masserman, Bill Myers, Joel Panchot,
　　Sandi Resnick, and William Teel.
　　■ Wintellectuals Finally, I’d like to thank the members of my extended Wintellect family for being patient as I took time away from the business to work on this project. In particular, I’d like to thank Jim Bail, Jason Clark, Paula Daniels, Peter DeBetta, Sara Faatz, Todd Fine, Lewis Frazer, Dorothy McBay, Jeff Prosise, John Robbins, and
　　Justin Smith.
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