Question

Hi i’m trying to create an Interactivity.Behavior to load a program’s icon in the background. The following is the code but it gave me The calling thread cannot access this object because a different thread owns it..

protected override void OnAttached()
    {
        base.OnAttached();

        if (!string.IsNullOrEmpty(Url))
        {
            Icon ico = Icon.ExtractAssociatedIcon(Url);
            if (ico != null)
            {

               taskScheduler = TaskScheduler.FromCurrentSynchronizationContext();
                Task.Factory.StartNew(() => {
                                                MemoryStream ms = new MemoryStream();
                                                ico.ToBitmap().Save(ms, ImageFormat.Png);
                                                ms.Position = 0;
                                                BitmapImage bi = new BitmapImage();
                                                bi.BeginInit();
                                                bi.StreamSource = ms;
                                                bi.EndInit();
                                                return bi;
                                            }).ContinueWith((t) => AssociatedObject.Source = t.Result, taskScheduler);




            }

        }
    }

Answer 1

WPF objects (anything that descends from DispatcherObject) are thread-affine — normally they can only be used on the thread that created them. This includes BitmapImage objects. If you create the BitmapImage on a background thread, then it can only be used from that background thread — which means that the UI thread will get an error when it tries to display the bitmap.

However, BitmapImage descends from Freezable. Freezable has a Freeze method that will make the instance read-only. And per the “Freezable Objects Overview” on MSDN:

A frozen Freezable can also be shared across threads, while an unfrozen Freezable cannot.

So if you add a call to bi.Freeze(); just before you return the image from your background task, then you should be able to use the image successfully from your UI thread.