In the past, compiler writers have put code to support the execution of programs into a runtime system. Instead of providing a different runtime system for each programming language, the .NET Framework provides a runtime system that is used by all of the languages that are targetted at the .NET Framework. This is called the Common Language Runtime (or CLR). Code that targets the CLR is called managed code.

Microsoft are providing several .NET compilers: Managed C++, Visual Basic.NET, JScript and C#. In addition, other people/companies are producing .NET compilers for other languages including COBOL, Eiffel, Haskell, ML, Perl, Python, Scheme and Smalltalk.

A .NET compiler writer can rely on the CLR for a large number of tasks, including:

• creating new types;
• creating and initializing of objects;
• tracking references to objects and providing garbage collection;
• handling the calling of methods (including virtual methods);
• managing the access to array elements;
• providing support for exceptions and exception handling.

All of the .NET languages have compilers that generate code written in an intermediate language called MSIL (or IL). A file containing MSIL instructions can be run on any platform so long as the operating system for that platform hosts the CLR engine. Currently, a CLR engine is available for Windows XP, Windows 2000, Windows NT 4.0, Windows 98 and Windows Me.