internal EventKey(WeakEventManager manager, object source)
 {
     _manager = manager;
     _source = source;
     _hashcode = unchecked(manager.GetHashCode() + RuntimeHelpers.GetHashCode(source));
 }
 internal EventKey(WeakEventManager manager, object source, bool useWeakRef)
 {
     _manager = manager;
     _source = new WeakReference(source);
     _hashcode = unchecked(manager.GetHashCode() + RuntimeHelpers.GetHashCode(source));
 }