No, as far as I know, Microsoft does not contractually obligate developers to use their tools. The .NET CLI is an ECMA standard, and there have been a few alter

No, as far as I know, Microsoft does not contractually obligate developers to use their tools. The .NET CLI is an ECMA standard, and there have been a few alternative languages developed for it (same as the JVM). I'm not aware of anything that would stop you from writing an XNA game using Boo, for example.