In 2008, Visual Studio team finally offers you some .NET Framework source code under a special license term to enhance your debugging experience.
So, try it if you are not going to contribute to Mono and if you have Visual Studio 2008 non-Express editions installed. Because the procedure is very complex, I can ensure I will never touch the source code. I don’t want to bind myself to Microsoft’s implementation of CLR at all until it is completely free (like Java VM).
Remember to check the license term before you touch the code.