/usr/lib/mono/4.5/Mono.C5.dll is in libmono-c5-1.1-cil 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Mono.C5/1.1.1.0__ba07f434b1c35cbd/Mono.C5.dll
/usr/lib/mono/4.5/../gac/Mono.C5/1.1.1.0__ba07f434b1c35cbd/Mono.C5.dll