public void ReadOnlyObservableCollectionCount() { var ints = new ObservableCollection <int>(); var source = new ReadOnlyObservableCollection <int>(ints); var values = new List <string>(); using (source.ObservePropertyChangedSlim("Count", signalInitial: false) .Subscribe(x => values.Add(x.PropertyName))) { CollectionAssert.IsEmpty(values); ints.Add(1); CollectionAssert.AreEqual(new[] { "Count" }, values); ints.Add(2); CollectionAssert.AreEqual(new[] { "Count", "Count" }, values); } }