This sixth iteration of the C# language is provided by the Roslyn compiler. This compiler came out with version 4.6 of the .NET Framework, however it can generate code in a backward compatible manner to allow targeting earlier framework versions. C# version 6 code can be compiled in a fully backwards compatible manner to .NET 4.0. It can also be used for earlier frameworks, however some features that require additional framework support may not function correctly.
The sixth version of C# was released July 2015 alongside Visual Studio 2015 and .NET 4.6.
As well as adding some new language features it includes a complete rewrite of the compiler. Previously
csc.exe was a native Win32 application written in C++, with C# 6 it is now a .NET managed application written in C#. This rewrite was known as project "Roslyn" and the code is now open source and available on GitHub.