Basics of .NET

Common Language Runtime (CLR)

The core of the Microsoft .NET technology is the Common Language Runtime (CLR) environment. This environment enables programmers to create programs using a multitude of programming languages and run them on any platform that supports the CLR. The idea of the CLR is to provide a middle layer of Application Program Interfaces (APIs) that operate between the low-level Windows Win32 API functions and the application program code. By providing a common middle layer, Microsoft has given a larger number of application languages access to core Windows technologies (such as network support).

The layout of how application programs run in the CLR environment is shown in Figure. High-level applications written in various .NET languages, such as Visual Basic .NET, Visual C⁺⁺ .NET, Visual J# .NET, and of course Visual C# .NET, are compiled into a special intermediate language called Microsoft Intermediate Language (MSIL). The MSIL code is interpreted by the CLR as the program runs; MSIL runs on the host operating system as a normal executable program. Of course, legacy programs that do not use the CLR can still directly access the low-level Windows Win32 APIs as before.

Figure: The Common Language Runtime (CLR) environment

This CLR model also pertains to other operating systems. Because the CLR is ported to other operating systems, .NET programs will be able to run without recompiling them on the new host systems. Currently, Microsoft supports the Shared Source Common Language Interface (CLI) project (nicknamed Rotor), which ports the CLR environment to the FreeBSD operating system. It is expected that the Rotor project will branch out to other operating systems as well. At the time of this writing you can get more information about Rotor at the following website:

http://msdn.microsoft.com/downloads/default.asp?URL=/downloads/sample.asp?url=/msdn-files/027/001/901/msdncompositedoc.xml

For programs to run in the CLR, they must be compiled into the special MSIL format. The .NET C# compiler is used to convert C# language programs to MSIL code that runs in the CLR environment. The next section describes the MSIL code.

MSIL Code

When you compile a C# program, it produces an executable file. However, this executable file is different from the ones you may be used to seeing produced from other Windows compilers. Instead of a low-level assembly program that can run directly in Windows, this executable file contains two parts:

• A stub assembly language program to start the CLR compiler

• The MSIL code of the compiled application

The stub program starts the CLR just-in-time (JIT) compiler, which compiles the MSIL program code to native Win32 code that can be run on the system. Unlike native Windows applications, which interact directly with the low-level Win32 API system, .NET applications rely on the .NET Framework CLR to run. Running a .NET application on a system that does not have the .NET Framework installed will produce an error message like the one in Figure. The .NET Framework is crucial to any .NET application, whether it is running on a Windows workstation or server. Without it the MSIL code cannot run. Any Windows workstation or server expected to run .NET programs must have the .NET Framework installed.

Figure: Trying to run a .NET application without the .NET Framework

Microsoft is committed to ensuring the .NET Framework will be installed on all future versions of the Windows OS. However, older versions of Windows must have the .NET Framework manually installed. The following section includes instructions for installing the .NET Framework for a developer environment to allow you to create, compile, and run .NET applications.