Microsoft is estopped by law from asserting patents on C#. That's one of the benefits of submitting specifications to a standards-granting organization.

The EFF should know this, but then again, having seen some of their legal work in the past, I'm surprised that they ever manage to win a case.

Edit Estoppel is a legal doctrine (which applies to most areas of law), which means that MS has performed certain actions (in this case, submitting the C# specification to the standards body) which would cause 3rd parties to reasonably assume that MS is waiving (patent) protections (or other legal rights in the matter at issue).