Provide a C# 1.0 compatibility mode.
If you're using the command line parser, you can say
"./csharp --compatibility=1.0 file.cs"
and the parser lists 2.0-only features as errors.

Also, implement the non-specified but MS/Mono-supported
fixed size buffer array declarations, which improves
the passed/failed test case ratio so that only
2 of 17050 files deliver errors when they shouldn't.

Also, let modifier flags be unsigned instead of signed ints.