Mono runtime - compiler compatibility for .NET 3.5

 Mono is a platform for running and developing applications based on the
 ECMA/ISO Standards. Mono is an open source effort led by Xamarin.
 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.
 .
 This package contains files required to compile applications against an older
 version of .NET, using Mono 4.0 or above, which only include the latest
 version of the class library as-is. This is important for producing apps for
 wider distribution