I'm working on a custom WPF UserControl and having an issue with one of my DependencyProperties. 

So I built a test scenario that looks like this. In the Custom Control..

```
public static readonly DependencyProperty MyCollectionItemsSourceProperty = DependencyProperty.Register("DynamicHeaderItemsSource", typeof(IEnumerable), typeof(TestUserControl1),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender, new PropertyChangedCallback(OnMyCollectionItemsSourceChanged))); 

public IEnumerable MyCollectionItemsSource
{
    get { return (IEnumerable)GetValue(MyCollectionItemsSourceProperty ); }
    set { SetValue(MyCollectionItemsSourceProperty , value); }
}

protected static void OnMyCollectionItemsSourceChanged(DependencyObject property, DependencyPropertyChangedEventArgs args)
{
    System.Diagnostics.Debug.WriteLine("MyCollection Updated");           
}

```

In my test window's code behind:

```
public ObservableCollection<string> MyTestStrings { get; set; }

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    MyTestStrings.Add("First");
    MyTestStrings.Add("Second");
    MyTestStrings.Add("Third");
}

```

And in my test window's XAML:

```
<Grid>
    <local:TestUserControl1 MyCollectionItemsSource="{Binding MyTestStrings}">
</Grid>

```

The problem is, I never get a notification of any type when the underline collection changes. The OnMyCollectionItemsSourceChanged only ever gets called once: at the beginning when the binding is set. What am I missing?