What language is .NET Framework written in?

The CLI/CLR is written in C/C++ and assembly. Almost all of the .NET framework classes are written in C# > compiled to IL, which runs in the CLR. If you crack open a framework library in Reflector, class, you may see an attribute such as [MethodImpl(MethodImplOptions.InternalCall)] which delegates the call to the CLI.

.NET was fully written in C and C++ because the base was in assembly language. Integration of assembly with C is much easier compared to newer languages.

There are parts of the .NET Framework that are open-source, like ASP.NET MVC, and it's written in C#.

Since the .NET Framework represents many assemblies, I'd presume that different libraries are written in different languages. As long as the language is CLR-compliant, it can be used to build parts of the framework.

All said, though; I'd assume the lion's share of the .NET Framework is written in C++ and C#.