Windows Phone C#, UnauthorizedAccessException when creating BitmapImage

Question

I am trying to download image from Internet using async call like this:

    private void DoGetAlbumart(object sender, DoWorkEventArgs e)
    {
        string req = (string)e.Argument;
        WebClient wc = new WebClient();
        wc.OpenReadCompleted += new OpenReadCompletedEventHandler(ReadWebRequestCallback);
        wc.OpenReadAsync(new Uri(req)); 

    }

    void ReadWebRequestCallback( object sender, OpenReadCompletedEventArgs e)
    {
        if (e.Error == null && !e.Cancelled)  
        {  
            try  
            {  
                BitmapImage image = new BitmapImage();  
                image.SetSource(e.Result);  
                SecondTile.Source = image;  
            }  
            catch (Exception ex)  
            {  
            }  
        }  
        else  
        {  
        }  
    }

It seems that when breakpoint hits at BitmapImage image = new BitmapImage(), I got the following exception:

ex = {System.UnauthorizedAccessException: Invalid cross-thread access. at MS.Internal.XcpImports.CheckThread() at System.Windows.DependencyObject..ctor(UInt32 nativeTypeIndex, IntPtr constructDO) at System.Windows.Media.Imaging.BitmapImage..ctor()

What else can I try to get rid of this error?

Answer 1

Callback methods run in background threads, not the UI thread. Unfortunately, BitmapImages can only be instantiated in the UI thread. If you need to access the UI thread from a callback, try the following:

Deployment.Current.Dispatcher.BeginInvoke(() =>
   {
      BitmapImage image = new BitmapImage();
      image.SetSource(e.Result);  
      SecondTile.Source = image;  
   });