public void Perform(Collections.ObservableHashSet <TItem> target)
 {
     foreach (TItem item in _items)
     {
         target.Add(item);
     }
 }
 public void Perform(Collections.ObservableHashSet <TItem> target)
 {
     if (_keyResolved)
     {
         target.Add(_key);
     }
 }
 public override void Perform(Collections.ObservableHashSet <TItem> target)
 {
     target.Add(_addedItem);
 }
 public override void Rollback(Collections.ObservableHashSet <TItem> target)
 {
     target.Add(_item);
 }