title: Dispatcher throws InvalidOperationException on Messagebox.Show in Textchanged event
body: <p>First of all this is the errorlog entry on my error</p>
  
  <p><code>crash program @ 15-9-2011 15:01:30error:System.InvalidOperationException: Dispatcher processing has been suspended, but messages are still being processed.
     at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean&amp; handled)</code> </p>
  
  <p>Anyway code:</p>
  
  <pre><code>private void TB_postcode_cijfers_TextChanged(object sender, TextChangedEventArgs e){
  if (TB_postcode_cijfers.Text != string.Empty || TB_postcode_cijfers.Text.Length &gt; 0)
  {
      LBL_postcode.Content = Postcode_cijfers + Postcode_letters;
      if (TB_postcode_cijfers.Text.Length == 4 &amp;&amp; TB_postcode_letters.Text.Length == 2)
      {
          if (!ZoekOpPostcode(Injectioncheck(TB_postcode_cijfers.Text + TB_postcode_letters.Text)))
          {
              //MessageBox.Show("Geen resultaat gevonden, " + errortext);
              if (MessageBox.Show("Geen resultaat gevonden, " + errortext + ".\n Wilt u overschakelen naar handmatig? ", "Handmatig?", MessageBoxButton.YesNo) == MessageBoxResult.Yes)
              {
                  RB_handmatig.IsChecked = true;
              }
              else
              {
                  //
              }
          }
      }
  }}
  </code></pre>
  
  <p>So on the messagebox.show method.
  this only happens when the user switches read mode to edit mode on my form.
  this involves collapsing en showing some labels and ui controls.</p>
  
  <p>if the event fires from userinput everything is fine.
  What I whant to know:
  Why does the textchanged event fire when hiding and showing a few controls.
  What can i do to prevent this error?</p>
  
  <p>EDIT:
  the code above is in a custom wpf control. placed in a winforms project/form     </p>