C# in linux environment

Learn Mono.

The Mono Project is an open development initiative sponsored by Novell to develop an open source, UNIX version of the Microsoft .NET development platform. Its objective is to enable UNIX developers to build and deploy cross-platform .NET Applications. The project implements various technologies developed by Microsoft that have now been submitted to the ECMA for standardization.

You can use the MonoDevelop IDE.

MonoDevelop is a free GNOME IDE primarily designed for C# and other .NET languages.

Mono is an open source .NET compiler, runtime and library.
Monodevelop is an open source C# IDE, primarily intended for linux development. It includes a GUI designer.