To be fair, like no programming language is not ‘open source’. A language could only be closed source if there’s only one compiler which is closed source. And in contrast to other languages like Python, which has the official interpreter, cython, PyPy, etc., C++, which has GCC, Clang, Mingw, C# only has the official M$ compiler provided by VS or the inofficial Mono. So C# is actually a lot more closed than nearly any other language.