public void Contains_ObservableSourceElementAdded_Update() { var update = false; var coll = new ObservableCollection <int>() { 1, 2, 3 }; INotifyEnumerable <int> notifiable = coll.WithUpdates(); var test = Observable.Expression(() => notifiable.Contains <int>(4)); test.ValueChanged += (o, e) => { update = true; Assert.IsFalse((bool)e.OldValue); Assert.IsTrue((bool)e.NewValue); }; Assert.IsFalse(test.Value); Assert.IsFalse(update); coll.Add(4); Assert.IsTrue(update); Assert.IsTrue(test.Value); }