"Dispatcher.BeginInvoke" seems not to be existing! in windowsphone 8.1

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Windows-Phone-7

I am trying to use the method "Dispatcher.BeginInvoke" in a Windows Phone 8.1 Universal App.

However, "Dispatcher.BeginInvoke" seems not to be existing!

"using System.Windows.Threading;" also does not exist!

I get the error message that the namesepace does not exist and I may be missing a reference assembly.

What shall I do?

I remember that I used this method in past when developing a Windows Phone 8.0 Project with VS 2013.

Now I have VS 2013 and a Windows 8.1 Universal App Project (but I am developing the Windows Phone 8.1 part of the project).


What is wrong?

Thanks for any hints!

public void XMPPClient_OnStateChanged(object sender, EventArgs e)
      {
          switch (xmppClient.XMPPState)
          {
              case XMPPState.Ready:

                  if (IsXmppSuccess)
                  {
                      this.Dispatcher.BeginInvoke(() =>
                      {
                          NavigationService.Navigate((new Uri("/Output.xaml?key=success", UriKind.Relative)));//error

                      });


                      //var dispatcher = Windows.UI.Core.CoreWindow.GetForCurrentThread().Dispatcher;
                      //var ignored = dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
                      //{
                      //    // update on ui thread
                      //});



                  }