Wei-Meng Lee - C# 2008 Programmer's Reference

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The .NET Framework is a development framework created by Microsoft to enable developers to build applications that run on Microsoft (and other) platforms. Understanding the basics of the .NET Framework is essential because a large part of C# development revolves around using the classes in that framework.

This chapter explains the key components in the .NET Framework as well as the role played by each of the components. In addition, it examines the relationships among the various versions of the Framework, from version 1.0 to the latest 3.5.

The .NET Framework has two components:

□ Common Language Runtime

□ .NET Framework class library

The Common Language Runtime (CLR) is the agent that manages your .NET applications at execution time. It provides core services such as memory, thread, and resource management. Applications that run on top of the CLR are known as managed code; all others are known as unmanaged code.

The .NET Framework class library is a comprehensive set of reusable classes that provides all the functionalities your application needs. This library enables you to develop applications ranging from desktop Windows applications to ASP.NET web applications, and Windows Mobile applications that run on Pocket PCs.

The Common Language Runtime (CLR) is the virtual machine in the .NET Framework. It sits on top of the Windows operating system (Windows XP, Windows Vista, Windows Server 2008, and so on). A .NET application is compiled into a bytecode format known as MSIL (Microsoft Intermediate Language). During execution, the CLR JIT ( just-in-time) compiles the bytecode into the processor's native code and executes the application. Alternatively, MSIL code can be precompiled into native code so that JIT compiling is no longer needed; that speeds up the execution time of your application.

The CLR also provides the following services:

□ Memory management/garbage collection

□ Thread management

□ Exception handling

□ Security

.NET developers write applications using a .NET language such as C#, VB.NET, or C++. The MSIL bytecode allows .NET applications to be portable (at least theoretically) to other platforms because the application is compiled to native code only during runtime.

At the time of writing, Microsoft's implementation of the .NET Framework runs only on Windows operating systems. However, there is an open-source implementation of the .NET Framework, called "Mono," that runs on Mac and Linux.

Figure 1-1 shows the relationships between the CLR, unmanaged and managed code.

Figure 1-1

The .NET Framework class library contains classes that allow you to develop the following types of applications:

□ Console applications

□ Windows applications

□ Windows services

□ ASP.NET Web applications

□ Web Services

□ Windows Communication Foundation (WCF) applications

□ Windows Presentation Foundation (WPF) applications

□ Windows Workflow Foundation (WF) applications

The library's classes are organized using a hierarchy of namespaces. For example, all the classes for performing I/O operations are located in the System.IOnamespace, and classes that manipulate regular expressions are located in the System.Text.RegularExpressionsnamespace.