Question: Was C# compiler written in C++?

Answer: 
Yes.

The Mono C# compiler is written in C#.
The .NET framework was written in Simple Managed C (SMC)

**History** 

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> During the development of the .NET Framework, the class libraries were originally written using a managed code compiler system called Simple Managed C (SMC).In January 1999, Anders Hejlsberg formed a team to build a new language at the time called Cool, which stood for "C-like Object Oriented Language".[16] Microsoft had considered keeping the name "Cool" as the final name of the language, but chose not to do so for trademark reasons. By the time the .NET project was publicly announced at the July 2000 Professional Developers Conference, the language had been renamed C#, and the class libraries and ASP.NET runtime had been ported to C#.
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From: <http://en.wikipedia.org/wiki/C_Sharp_%28programming_language%29>