/usr/lib/mono/4.0/WindowsBase.dll is in libmono-windowsbase4.0-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/WindowsBase/4.0.0.0__31bf3856ad364e35/WindowsBase.dll