Mono C# 4.0 compiler for CLI 4.0 This is the Mono C# (C-Sharp) 4.0 compiler, a platform-independent compiler which produces CIL (Common Intermediate Language) binary executables. This compiler targets the CLI 4.0 runtime version. Mono is a platform for running and developing applications based on the ECMA/ISO Standards. Mono is an open source effort led by Novell. Mono provides a complete CLR (Common Language Runtime) including compiler and runtime, which can produce and execute CIL (Common Intermediate Language) bytecode (aka assemblies), and a class library.