C# AutoResetEvent WaitOne stopped by dispatcher

Question

private void WaitForDriveToBecomeReady()
{
    AutoResetEvent syncEvent = new AutoResetEvent(false); //set wait signal to use later

    //dispatcher to be able to change stuff in xaml from within thread
    Action action1 = new Action(delegate() { grdMain.Children.Add(notification); });
    Action action2 = new Action(delegate() { grdMain.Children.Remove(notification); });
    Thread restoreThread1 = new Thread(()=>{
        grdMain.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Background, action1); //show a notification

        Thread.Sleep(1500); //sleep a bit...

        grdMain.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Background, action2); //hide a notification

        syncEvent.Set(); //signal to continue at *.WaitOne()
    });
    restoreThread1.Start();

    syncEvent.WaitOne(); //let main thread wait until *.Set(); is called
}

The above code works perfect IF you comment out the two grdMain.Dispatcher.Invoke(...);. It also works perfekt if you comment out the *.Set(); and *.WaitOne(); But WHYYYY? I need both ^^. I don't get it...

Answer 1

I finally had time to continue read more about async and await. Thanks @Jacob for pointing out what was the problem.

Here is my working async code for a toast notification that appears as long as you don't connect your drive.

//the method
public async Task WaitForDriveAsync(string path, string waitingToastText)
{
    int width = 300;
    int height = 125;
    TextBlock toastTextBlock = new TextBlock() { Text = waitingToastText, HorizontalAlignment = HorizontalAlignment.Center, VerticalAlignment = VerticalAlignment.Center, FontSize = 23, Width = (width - 15), TextWrapping = TextWrapping.WrapWithOverflow };
    Grid notification = new Grid();
    notification.Width = width;
    notification.Height = height;
    notification.Background = Brushes.Red;
    notification.Margin = new System.Windows.Thickness(0, 27, 0.4, 0);
    notification.VerticalAlignment = VerticalAlignment.Top;
    notification.HorizontalAlignment = HorizontalAlignment.Right;
    notification.Children.Add(toastTextBlock);

    grdMain.Children.Add(notification);

    while (!Directory.Exists(path))
    {
        await Task.Delay(1000);
    }

    grdMain.Children.Remove(notification);
}

//to call it
private async void btnBackupNow_Click(object sender, RoutedEventArgs e)
{
    await WaitForDriveAsync(@"B:\", "Please connect your drive.");
}