My company has a large C# code base. Well over half of this code is our core engine for creating, reading, modifying, calculating
I made a comment that should really have been an answer:
It is just not technically possible at this point to implement the C# spec on the JVM. C# supports pointers, unsigned types, user defined value types, true generics, passing by reference, and loads of other things. For a very C#-like JVM language, check out Stab.
Mainsoft fakes it. I am sure it takes enormous effort on their part.