Changes made in Entity_Saving method from a dispatcher are not saved
===

Within my LightSwitch application (VS2012RC) I have implemented the `Entity_Saving` operation.  Within this operation I am building up some data to go back into the entity, however this is built from a CustomControl.  I therefore have to switch to using the Dispatcher on the control to prevent an `UnauthorisedAccessException`, however when I do this the save isn't persisted.  I have tried to use an AutoResetEvent in order to prevent the Saving method from completing until the property has been updated, but it appears to ignore it.

The data it's building is actually an image that's calculated based on several properties within the entity, therefore I would prefer to avoid rebuilding the image every time one of the properties changes, and just build it at the point the entity is saved.

In the screen code:

        partial void EntityDetail_Saving(ref bool handled)
        {
            this.Entity.SomeProperty = "abc"; // this works fine

            AutoResetEvent autoResetEvent = new AutoResetEvent(false);
            CustomControl.FetchData(data =>
                {
                    // this is not saved and following the 
                    // save the entity is left in a state of changed
                    this.Entity.AnotherProperty = data;
                    autoResetEvent.Set();
                });
            autoResetEvent.WaitOne();
        }

In the custom control:

        public void FetchData(Action<string> callback)
        {
            Dispatcher.BeginInvoke(() =>
                {
                    string data = ""; // do some calculations here
                    if (callback != null) callback(data);
                });
        }