The mono C# compiler has been written in C# from the start, and this proved to be very helpful. I remember at the conference where Anders Hejlsberg first discu

The mono C# compiler has been written in C# from the start, and this proved to be very helpful. I remember at the conference where Anders Hejlsberg first discussed Roslyn (which encompasses rewriting the microsoft C# compiler in C#), Miguel de Icaza was able to show some of the same ideas he was discussing actually already running on Mono. I don't think Miguel even knew ahead of time; it was just something the mono people had done.