I think you may take a look at the Shared Source CLI.
Shared Source CLI Provides Source Code for a FreeBSD Implementation of .NET http://msdn.microsoft.com/msdnmag/issues/02/07/SharedSourceCLI/
The Shared Source CLI ships with source code to five assemblies:
Mscorlib.dll, System.dll, System.xml.dll, System.runtime.remoting.dll, and
System.runtime.serialization.formatters.soap.dll.