Internal Program Version: Mono  6.12.0

The Mono Project is an open development initiative that is working to
develop an open source, Unix version of the .NET development platform.
Its objective is to enable Unix developers to build and deploy
cross-platform .NET applications. The project will implement various
technologies that have been submitted to the ECMA for standardization.

Mono's Windows Forms implementation.

http://www.mono-project.com