Roslyn API basically lets you plug in to the C# compiler or use parts of it so that you can get the same view of the abstract syntax tree, typechecking, flow analysis, etc. that the compiler does. So it's a way to work with C# code at that level rather than just via textual manipulation. I'm less familiar with other languages but I think many have compilers with analogous capabilities, I know Haskell has the GHC API and I think the LLVM-based Clang (C++) and Rust compilers let you do similar things