The "old" compiler which is written in C/C++, I don't think that it's open source though. The modern compiler - Roslyn - was explicitly re-written in C#, and i

The "old" compiler which is written in C/C++, I don't think that it's open source though. The modern compiler - Roslyn - was explicitly re-written in C#, and is currently the main compiler (and has been for about three years now).