If one of these fields is accidentally reassigned, it's possible for threads to be oblivious to an existing lock. By making the fields readonly, we will be notified at compile-time if we inadvertently redefine one of these fields.
git-svn-id: https://duplicati.googlecode.com/svn/trunk@1323 59da171f-624f-0410-aa54-27559c288bec