You could always get the .NET Framework Library Source code with Lutz Roeder's .NET Reflectorand Reflector.FileDisassembler but now Microsoft went and licensed it to us. They also included comments and made it easy to debug into it, which is really cool.
The source is Reference Licensed, which means you can read it, but you can't use it. So unlike Delphi's VCL/RTL source code, you can't make an enhancement in the framework and recompile and ship. Nor can you build on the code in new and creative ways.
Interestingly, they changed the Reference License to specifically exclude anyone working from the Mono project to look at the source code (or to forfeit their legal ability to contribute if they do). But if the .NET Framework clone is for the windows platform, you can look all you want. Under the unmodified license, if you looked at the code, you couldn't build similar functionality on any platform. Now they only prohibit it on non-Windows platforms. So I wonder . . . If someone looked at the code to work on a Windows port of Mono, it would seem that would be permitted under the license. Then someone could take the new inspired code, and use that in a non-Windows port of the Windows port of Mono. Hmmm . . . I am not a lawyer, nor have a read the license, but it is an interesting proposition.