/usr/lib/mono/4.5/System.Xaml.dll is in libmono-system-xaml4.0-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.Xaml/4.0.0.0__b77a5c561934e089/System.Xaml.dll