Ups, I've done it. It's fubar [1] now.

However it's possible to send an error report using the errorreport switch: csc *.cs /errorreport:prompt

Actually I found why the compiler screw up. I use a lot of plattform invokes and because I am lazy I use a constant with the library name.

private const string LIB = "MyNativeDll.dll";

[DllImport(LIB)]
//...

While changing the code, I removed the const modifier by accident, which causes the compiler to fail...

[1] dict.tu-chemnitz.de
[2] msdn.microsoft.com