My UI thread is blocked when await Dispatcher.BeginInvoke... (using WPF and WCF)

Question

I have a WPF window that creates and starts a timer in its constructor. The timer elapsed event triggers a method (SyncPTUpdate) which uses BeginInvoke to place a call to another method (PTProgressUpdateInThread) onto the Window's thread. This then calls a WCF call asynchronously (using the TAP pattern, auto-generated by VS 2013).

When I make the WCF call artificially long in duration (using thread.sleep in the server component), the UI of my WPF application freezes. Not initially, but after a few seconds have gone by.

Where am I going wrong?

public delegate void PTProgressDelegate();

// this method is called from a periodically firing timer (System.Timers)
private async void SyncPTUpdate(object sender, ElapsedEventArgs e)
{
    await this.Dispatcher.BeginInvoke(DispatcherPriority.Normal, new PTProgressDelegate(PTProgressUpdateInThread));
}

private async void PTProgressUpdateInThread()
{
    PTMapFieldClient = new FieldClient();
    ServiceField.BlokPTProgress PTProgressFromServer = await PTMapFieldClient.GetPTProgressAsync(variousparametershere);
    PTMapFieldClient.Close();

    // now use the results of the WCF call to update the Window UI
    //...
}

Answer 1

Dispatcher.BeginInvoke() is often presented as a way to do things async, with the benefit of not having to create/involve another thread.

But that means it is only beneficial for small, lightweight jobs. Best thing here is to push the call(s) to PTProgressUpdateInThread() to the ThreadPool. Or yuse a threaded Timer.
You're not using the results after await anyway.