[ystad]

The .NET source https://github.com/microsoft/referencesource https://github.com/microsoft/dotnet Unity reference source https://github.com/Unity-Technologies/UnityCsReference

The above are example of production code. I use these as references all the time

Awesome c# https://github.com/quozd/awesome-dotnet

[dubiousconst281]

ReferenceSource is the code from the Windows framework and I'm pretty sure it should be considered deprecated at this point. The runtime and core libraries are now at https://github.com/dotnet/runtime, kinda weird that this is not directly linked in the home page repo.

[runevault]

I'd be careful about recommending Awesome dotnet, their list has some deprecated codebases like NancyFx (I'd bet others but I know that one)