/usr/bin/csharp is in mono-csharp-shell 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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#!/bin/sh exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/4.5/csharp.exe "$@"