ObservableCollection that also monitors changes on the elements in collection

Made a quick implementation myself:

public class ObservableCollectionEx<T> : ObservableCollection<T> where T : INotifyPropertyChanged
{
    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        Unsubscribe(e.OldItems);
        Subscribe(e.NewItems);
        base.OnCollectionChanged(e);
    }

    protected override void ClearItems()
    {
        foreach(T element in this)
            element.PropertyChanged -= ContainedElementChanged;

        base.ClearItems();
    }

    private void Subscribe(IList iList)
    {
        if (iList != null)
        {
            foreach (T element in iList)
                element.PropertyChanged += ContainedElementChanged;
        }
    }

    private void Unsubscribe(IList iList)
    {
        if (iList != null)
        {
            foreach (T element in iList)
                element.PropertyChanged -= ContainedElementChanged;
        }
    }

    private void ContainedElementChanged(object sender, PropertyChangedEventArgs e)
    {
        OnPropertyChanged(e);
    }
}

Admitted, it would be kind of confusing and misleading to have the PropertyChanged fire on the collection when the property that actually changed is on a contained element, but it would fit my specific purpose. It could be extended with a new event that is fired instead inside ContainerElementChanged

Thoughts?

EDIT: Should note that the BCL ObservableCollection only exposes the INotifyPropertyChanged interface through an explicit implementation so you would need to provide a cast in order to attach to the event like so:

ObservableCollectionEx<Element> collection = new ObservableCollectionEx<Element>();
((INotifyPropertyChanged)collection).PropertyChanged += (x,y) => ReactToChange();

EDIT2: Added handling of ClearItems, thanks Josh

EDIT3: Added a correct unsubscribe for PropertyChanged, thanks Mark

EDIT4: Wow, this is really learn-as-you-go :). KP noted that the event was fired with the collection as sender and not with the element when the a contained element changes. He suggested declaring a PropertyChanged event on the class marked with new. This would have a few issues which I’ll try to illustrate with the sample below:

  // work on original instance
  ObservableCollection<TestObject> col = new ObservableCollectionEx<TestObject>();
  ((INotifyPropertyChanged)col).PropertyChanged += (s, e) => { Trace.WriteLine("Changed " + e.PropertyName); };

  var test = new TestObject();
  col.Add(test); // no event raised
  test.Info = "NewValue"; //Info property changed raised

  // working on explicit instance
  ObservableCollectionEx<TestObject> col = new ObservableCollectionEx<TestObject>();
  col.PropertyChanged += (s, e) => { Trace.WriteLine("Changed " + e.PropertyName); };

  var test = new TestObject();
  col.Add(test); // Count and Item [] property changed raised
  test.Info = "NewValue"; //no event raised

You can see from the sample that ‘overriding’ the event has the side effect that you need to be extremely careful of which type of variable you use when subscribing to the event since that dictates which events you receive.