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private static System.Windows.Threading.DispatcherOperationCallback exitFrameCallback = new
             System.Windows.Threading.DispatcherOperationCallback(ExitFrame);

        private static Object ExitFrame(Object state)
        {
            System.Windows.Threading.DispatcherFrame frame = state as System.Windows.Threading.DispatcherFrame;

            frame.Continue = false;
            return null;
        }

        public static void DoEvents()
        {
            System.Windows.Threading.DispatcherFrame nestedFrame = new System.Windows.Threading.DispatcherFrame();

           
            System.Windows.Threading.DispatcherOperation exitOperation = System.Windows.Threading.Dispatcher.CurrentDispatcher.BeginInvoke(
                System.Windows.Threading.DispatcherPriority.Background, exitFrameCallback, nestedFrame);


            System.Windows.Threading.Dispatcher.PushFrame(nestedFrame);

            if (exitOperation.Status != System.Windows.Threading.DispatcherOperationStatus.Completed)
            {
                exitOperation.Abort();
            }
        }					

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  var max := 1000; //irgendein Wert für unsere Beispiel-Schleife
  progressBar1.Maximum := max; //Maximum der ProgressBar entsprechend setzen
  async begin
    for i : Integer := 0 to max do
    begin
      Thread.Sleep(100); //Stück der langen Berechnung

      Dispatcher.Invoke(DispatcherPriority.Normal, method; begin
        progressBar1.Value := i; //Statusbar aktualisieren
      end);
    end;
  end;					

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public class Haupklasse{
     public delegate void DoCalculateDelegate();
     public delegate void UpdateDisplayDelegate();

        public Haupklasse()
        {
            InitializeComponent();
        }

        private void btnDoSomething_Click(object sender, RoutedEventArgs e)
        {
         // Sonstiger Code
         ...
         // EOF Sonstiger Code

          DoCalculateDelegate fetcher = new DoCalculateDelegate(this.DoCalculate);

          fetcher.BeginInvoke(null, null);
         }

        public void DoCalculate()
        {
           for(i=0;i<anzahlberechnungen;i++)
           {

           // Hier grosse langandauernde Berechnung


           this.Dispatcher.BeginInvoke(DispatcherPriority.Normal,new UpdateDisplayDelegate(UpdateUserInterface));
           }
        }

        private void UpdateUserInterface()
        {
            // Hier kann man auf die Elemente der benutzeroberfläche Zugreiffen
     
        }
}