Edit `filetypes.cs` and change the compiler from `mcs` to `csc`? I keep the `filetypes.cs` on Geany installation directory intact. Instead of copy it into `%appdata%\geany\filedefs` and modify it here. It doesn't work. I tried to comment out `C#=*.cs;` in `%appdata%\geany\filetype_extensions.conf`. Nothing worked. It will always force `mcs` as the compiler.