Have worker thread update ObservableCollection that is bound to a ListCollectionView

While playing around with WPF, I tried to do some multithreading where I have a worker thread updating my ObservableCollection, while having a ListCollectionView of that ObservableCollection being shown on a ListBox.

It was surprising to see that I get a NotSupportedException thrown, with the message saying ‘This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread.’.  That doesn’t seem to make sense – In my mind, I understand how the thread that created the UI should be the one that handles all UI updates.  However, the data itself should be able to reside anywhere, and I should be able to update it however and whenever I want.

Looking for a solution, I created a class deriving from ObservableCollection thinking that I would just manually walk through the event’s invocation list.  Well, that didn’t quite work, since events are not accessible (other than for adding/removing delegates) to child classes.  Looking at the documentation, the CollectionChanged event in ObservableCollection is virtual – that means I can override it! Yeah!  I am glad someone at Microsoft decided to make that virtual.

So here’s the code that I created – the pain is I now have to rename all occurrences of ObservableCollection to this new class.  Oh well, at least making it work with threads isn’t too painful.

protected override void OnCollectionChanged(
    System.Collections.Specialized.NotifyCollectionChangedEventArgs e)

{
  // Be nice - use BlockReentrancy like MSDN said

  using (BlockReentrancy())

  {
    System.Collections.Specialized
        .NotifyCollectionChangedEventHandler eventHandler = CollectionChanged;

    if (eventHandler == null)

      return;

    Delegate[] delegates = eventHandler.GetInvocationList();

    // Walk thru invocation list

    foreach (System.Collections.Specialized
                 .NotifyCollectionChangedEventHandler handler in delegates)

    {
      DispatcherObject dispatcherObject = handler.Target as DispatcherObject;

      // If the subscriber is a DispatcherObject and different thread

      if (dispatcherObject != null && dispatcherObject.CheckAccess() == false)

      {
        // Invoke handler in the target dispatcher's thread

        dispatcherObject.Dispatcher.Invoke(DispatcherPriority.DataBind, handler,
                                           this, e);

      }

      else  // Execute handler as is

        handler(this, e);
    }
  }
}

Update:

There is a bug with the code where subscribers in the same thread won’t get called – the code above has been updated with the fix.

It also uses CheckAccess (available, but not shown via Intellisense as mentioned here)