There's a line in csharp-model.el that reads:
(add-to-list 'auto-mode-alist '("\\.cs" . csharp-mode))
it should be, instead:
(add-to-list 'auto-mode-alist '("\\.cs\\'" . csharp-mode))
the \\' in regexp-speak matches the end of the string. In other words, the pattern is a file that ends with ".cs". As it is, your pattern matches anything with a .cs in it, anywhere. So, a file like Alpha.cst.bin will match, and will be inserted into csharp-mode.
I tried reporting this via your bug database but got nowhere. see my other msg about that.
