/usr/bin/csharp is in mono-csharp-shell 2.10.8.1-1ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2
#!/bin/sh exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/4.0/csharp.exe "$@"