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

A change within the collection won't trigger the OnSpecialDaysChanged callback, because the value of the dependency property hasn't changed. If you need to react to detect changes with the collection, you need to handle the event CollectionChanged event manually:

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
    coll.CollectionChanged -= 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
    coll.CollectionChanged += SpecialDays_CollectionChanged;
  }
}

private static void SpecialDays_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // handle CollectionChanged
}