/usr/lib/mono/2.0/WindowsBase.dll is in libmono-windowsbase3.0-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/WindowsBase/3.0.0.0__31bf3856ad364e35/WindowsBase.dll