The original version of the C# compiler was written in C and C++, but today's version is indeed itself written in C#!
https://stackoverflow.com/questions/27513234/in-which-langua...
https://stackoverflow.com/questions/27513234/in-which-langua...
Its been a while, and I think it was gcc 7.x timeframe, and IIRC, you had to have at a min, gcc 4.x available to bootstrap, and it took something like 3 separate bootstrap iterations to get the final compilers was like 4-6 hours if memory serves.