title: WPF Dispatcher Invoke return value is always null
body: <p>I have a call to a method that returns a <code>UIElement</code> that I call using the <code>Dispatcher</code>, below is the code.</p>
  
  <p>However the return value of the <code>Dispatcher</code> invoke is always NULL, any ideas?</p>
  
  <pre><code>void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
  {
      var slides = (IList&lt;UIElement&gt;)e.Argument;
      var bmpSlides = new List&lt;UIElement&gt;();
      var imageService = new ImageService();
      int count = 0;
  
      foreach (UIElement slide in slides)
      {
          object retVal = slide.Dispatcher.Invoke(
              new ThreadStart(() =&gt; imageService.GenerateProxyImage(slide)));
          bmpSlides.Add(imageService.GenerateProxyImage(slide));
          _backgroundWorker.ReportProgress(count / 100 * slides.Count);
          count++;
      }
  
      e.Result = bmpSlides;
  }
  </code></pre>