This is not what you think if you thought that .NET Framework is not Open Sourced. But the good thing is now we can access the source of .NET Framework base libraries.
Well, since it’s not open source, you can’t port it to another platform (i.e. UNIX or Mainframe) or create your own fork of .NET Framework. But under Microsoft Shared Source INitialtives, you are able to have a look at the source code and undersand the behavior of the base class libraries. One idea I have is if you seen something you don’t like about the class library, you can always create your own version of it, and host it under your own Namespace, i.e. PatrickYong.XML. But before that, please have a look at the license agreement on what you can or cannot do.
Below is the list of libraries released according to Microsoft’s Reference Source Code Center.
Now you can also enable Visual Studio 2008 to debug inside the .NET Framework class libraries. Follow the instructions here.