I don't expect the C# compiler to understand F#, but in my opinion there should be language independent ways to create CLR assemblies. MSBuild (which calls the right compiler for each project) could also understand (and did iirc) compiler-independent project types that still produce assemblies, however, from mixed sources. Now the C# compiler would only be called for the C# sources.

> Basically a single assembly has to be written in the same language.

This is like saying a single js file has to come from only js files, still bundlers today merge all kinds of types in there - in a single sweep.

I am not writing this since I think mixing C#/F# is the future (well, it would be cool), but because it could allow the C# ecosystem to improve. Right now the best thing we have for metaprogramming in C# is Fody; and I personally think we could do better.

Hopefully this expressed my idea a little bit better.