Question

I am trying to load an image in the background and then update the UI. I have been playing with this all day and I don’t know what I am missing. I keep getting the following error:

‘The calling thread cannot access this object because a different thread owns it.’

I’ve hunted around following example after example, but I cannot seem to find an answer. I also wrapped the code that is touching the UI in another BeginInvoke.

Update 3: The moral of the story. ImageSource is not thread safe for access.

Update 2: This has got to be a simple solution :). I tried the cloning, but that didn’t result in success, but I did get a different error: ‘Exception has been thrown by the target of an invocation.’

Update 1: I tried the BackgroundWorker, but I am still getting the same error, but it is occurring on the brush.ImageSource.Height. Am I signaling the UI correctly? Any suggestions?

Here is my XAML:

<Window x:Class='Slideshow.Show' xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>     <DockPanel>         <Canvas Background='Black' Name='canvas'>             <Viewbox Name='background' Stretch='Uniform'>                 <Rectangle name='background' />             </Viewbox>         </Canvas>     </DockPanel> </Window> 

Here is some of the code behind:

namespace Slideshow {     public class Show      {         public Show()         {             BackgroundWorker bw = new BackgroundWorker();             bw.DoWork += new DoWorkEventHandler(bw_DoWork);             bw.RunWorkerCompleted +=                  new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);             bw.RunWorkerAsync();         }          void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)         {             BitmapSource bitmap = e.Result as BitmapSource;              if (bitmap != null)             {                 this.Dispatcher.BeginInvoke(DispatcherPriority.Normal                      (ThreadStart)delegate()                 {                     Image image = new Image();                     image.Source = bitmap;                     background.Child = image;                  });             }         }          void bw_DoWork(object sender, DoWorkEventArgs e)         {             BitmapSource bitmapSource = CreateBrush(GetRandomFile());             e.Result = bitmapSource;          }     } } 

Answer 1

What I tend to use the ThreadPool.QueueUserWorkItem to load the image, then when the operation completes I call back to the UI thread using the thread-safe Dispatcher object. The image source is not thread safe, you will have to use something like the JpegBitmapDecoder, there is also a PngBitmapDecoder.

For Example:

public Window() {     InitializeComponent();      ThreadPool.QueueUserWorkItem(LoadImage,          'http://z.about.com/d/animatedtv/1/0/1/m/simp2006_HomerArmsCrossed_f.jpg'); }  public void LoadImage(object uri) {     var decoder = new JpegBitmapDecoder(new Uri(uri.ToString()), BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.OnLoad);     decoder.Frames[0].Freeze();     this.Dispatcher.Invoke(DispatcherPriority.Send, new Action<ImageSource>(SetImage), decoder.Frames[0]); }  public void SetImage(ImageSource source) {     this.BackgroundImage.Source = source; }