/usr/lib/mono/2.0/System.Net.dll is in libmono-system-net2.0-cil 3.2.8+dfsg-4ubuntu1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/2.0/../gac/System.Net/3.5.0.0__b77a5c561934e089/System.Net.dll