The build system for C# (.NET anyway used synonymously but technically completely idependent) is MSBuild which doesn't care where the libraries come from as long as your project points to a place it can find and link them.
Discussion

a lot of microsoft side projects were not terrible, and p2p was popular in amongst it all, maybe the only virtue of microsoft
C# at least was outside of Microsoft, and I think the Rosolyn C# compiler is still maintained outside Microsoft by the original developer. He may be on MS payroll, but I think its still independent. MISL and .NET on the otherhand, FOSS at least, but yeah captured by MS.