ObservableCollection dependency property does not update when item in collection is deleted

Question

I have a attached property of type ObservableCollection on a control. If I add or remove items from the collection, the ui does not update. However if I replace the collecti

Answer 1

If you have a collection-type dependency property keep the following in mind:

If your property is a reference type, the default value specified in dependency property metadata is not a default value per instance; instead it is a default value that applies to all instances of the type. [...]
To correct this problem, you must reset the collection dependency property value to a unique instance, as part of the class constructor call.

(see MSDN Collection-Type Dependency Properties)

To answer Sam's question (I just ran into the same problem):

Make your CollectionChanged-handler non-static and unsubscribe/re-subscribe on instance-level.

private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  var calendar = d as RadCalendar;

  if (e.OldValue != null)
  {
    var coll = (INotifyCollectionChanged)e.OldValue;
    // Unsubscribe from CollectionChanged on the old collection of the DP-instance (!)
    coll.CollectionChanged -= calendar.SpecialDays_CollectionChanged;
  }

  if (e.NewValue != null)
  {
    var coll = (ObservableCollection)e.NewValue;
    calendar.DayTemplateSelector = new SpecialDaySelector(coll, GetSpecialDayTemplate(d));
    // Subscribe to CollectionChanged on the new collection of the DP-instance (!)
    coll.CollectionChanged += calendar.SpecialDays_CollectionChanged;
  }
}

private void SpecialDays_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // handle CollectionChanged on instance-level
}