/usr/lib/mono/4.0/Microsoft.CSharp.dll is in libmono-microsoft-csharp4.0-cil 2.10.8.1-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.0/../gac/Microsoft.CSharp/4.0.0.0__b03f5f7f11d50a3a/Microsoft.CSharp.dll