How is that different from any other OSS project? If MS wants, they could look at a projects source code and decide that they've violated a patent and sue them. To my knowledge they're yet to do this.
The difference is that Microsoft created .Net and the patents covering it together, so they're fairly narrow patents and it'd be a hell of a lot harder to find relevant prior art to use against them. (The same's true of OOXML, incidentally; Microsoft even tried to get a patent covering both it and ODF but the examiner narrowed it to just OOXML based on a huge amount of prior art dating back to SGML in the 80s.)
