/usr/bin/genxs is in mono-devel 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.
1 2
#!/bin/sh exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/4.5/genxs.exe "$@"