Not sure what you are looking for or mean by 'fully open source'. The .NET Framework libraries are open source, ASP.NET is open source, the C# compiler itself is closed is closed but it is an ECMA standard and there are open source implementations of it.
They're only open source in the sense you can view them, but you definitely cannot do anything with them, edit them, compile them, etc (from what i can determine). Its purely for reference, not use.
