Not sure about the core program. I know it uses a unity 3d engine. But the addon's that I have seen for added functionality are written in c#. If you haven't tried it, get the MechJeb addon for assists in launching/landing and transfering to other planets/moons. Also, it helps you keep from killing your kerbalnauts!! :)

KERBAL SPACE PROGRAM is much more FUN!!!! kerbalspaceprogram.com Free for demo. $18 to get in on the beta/alpha developemnt. All C# development. BUILD your own addon's in C#!!!