Unfortunately, this article conflates two things: the C# REPL, and the C# compiler-as-a-service. Both are part of Roslyn, which is a reimplementation of the C#

Unfortunately, this article conflates two things: the C# REPL, and the C# compiler-as-a-service. Both are part of Roslyn, which is a reimplementation of the C# compiler written in C#. (It was previously written in C++.) The REPL could have been implemented on top of the older C++ compiler. The C# REPL is something that Mono has had for a few years now. The compiler-as-a-service has some resemblance to what LLVM + Clang enable for a C compiler.