Microsoft .NET documentation: https://docs.microsoft.com/dotnet
The .NET API documentation: https://docs.microsoft.com/dotnet/api
The C# language design repository: https://github.com/dotnet/csharplang
The Roslyn repository: https://github.com/dotnet/roslyn
The C# ECMA standard: www.ecma-international.org/publications/standards/Ecma-334.htm
Stack Overflow: https://stackoverflow.com