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 (the class name I chose is ObservableCollectionEx) 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.
public class ObservableCollectionEx: ObservableCollection { // Override the event so this class can access it public override event System.Collections.Specialized.NotifyCollectionChangedEventHandler CollectionChanged;
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).