In looking around for C# 3d files, I came across
Purple#. It claims to be based entirely on shaders, and is made in pure C#. The source code is available from the web site, and it's GPL. I'm not overly experienced in 3D engines so I can't really judge its merit, but it was interesting to find one written from the ground up with C#. Axiom is C#, but it's a port of a C++ engine (Ogre), and not designed to take full advantage of C# and .NET.