/usr/lib/mono/4.5/System.Reactive.Windows.Threading.dll is in libmono-system-reactive-windows-threading2.2-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/System.Reactive.Windows.Threading/2.2.0.0__31bf3856ad364e35/System.Reactive.Windows.Threading.dll