"where do i get a thread-safe collectionview?" Code Answer

the following is an improvement on the implementation found by jonathan. firstly it runs each event handler on the dispatcher associated with it rather than assuming that they are all on the same (ui) dispatcher. secondly it uses begininvoke to allow processing to continue while we wait for the dispatcher to become available. this makes the solution much faster in situations where the background thread is doing lots of updates with processing between each one. perhaps more importantly it overcomes problems caused by blocking while waiting for the invoke (deadlocks can occur for example when using wcf with concurrencymode.single).

public class mtobservablecollection<t> : observablecollection<t>
{
    public override event notifycollectionchangedeventhandler collectionchanged;
    protected override void oncollectionchanged(notifycollectionchangedeventargs e)
    {
        notifycollectionchangedeventhandler collectionchanged = this.collectionchanged;
        if (collectionchanged != null)
            foreach (notifycollectionchangedeventhandler nh in collectionchanged.getinvocationlist())
            {
                dispatcherobject dispobj = nh.target as dispatcherobject;
                if (dispobj != null)
                {
                    dispatcher dispatcher = dispobj.dispatcher;
                    if (dispatcher != null && !dispatcher.checkaccess())
                    {
                        dispatcher.begininvoke(
                            (action)(() => nh.invoke(this,
                                new notifycollectionchangedeventargs(notifycollectionchangedaction.reset))),
                            dispatcherpriority.databind);
                        continue;
                    }
                }
                nh.invoke(this, e);
            }
    }
}

because we are using begininvoke, it is possible that the change being notified is undone before the handler is called. this would typically result in an "index was out of range." exception being thrown when the event arguments are checked against the new (altered) state of the list. in order to avoid this, all delayed events are replaced with reset events. this could cause excessive redrawing in some cases.