protected override void OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs args)
{
var olditems = (args.OldItems ?? new T[0]).OfType <T>();
_host.Remove(olditems);
var newitems = (args.NewItems ?? new T[0]).OfType <T>();
_host.Add(newitems);
_host.Update();
base.OnCollectionChanged(args);
}
