How to differentiate between button press and button long press in c# on .Net Framework 3.5

/// <summary>
/// Detect Mouse Long Click
/// </summary>
/// <param name="element">Framework</param>
/// <param name="duration">Set duration of long click</param>
/// <returns>true: Long Click false: Not Long Click</returns>
public static Task<bool> MouseDown(this FrameworkElement element, TimeSpan duration)
{
    DispatcherTimer timer = new DispatcherTimer();
    TaskCompletionSource<bool> task = new TaskCompletionSource<bool>();
    timer.Interval = duration;

    MouseButtonEventHandler touchUpHandler = delegate
    {
        timer.Stop();
        if (task.Task.Status == TaskStatus.Running)
        {
            task.SetResult(false);
        }
    };

    element.PreviewMouseUp += touchUpHandler;

    timer.Tick += delegate
    {
        element.PreviewMouseUp -= touchUpHandler;
        timer.Stop();
        task.SetResult(true);
    };

    timer.Start();
    return task.Task;
}

private async void btn_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {

        boolean isLongPress = await MouseDown(e.Source as Button,           TimeSpan.FromSeconds(0.4));
       if (isLongPress) {
          // long press
       } else {
        //short press
       }
    }

private bool mouseUp = false;
private const int holdButtonDuration = 2000; //milliseconds
private void btnTest_MouseDown(object sender, MouseEventArgs e)
{
    mouseUp = false;
    Stopwatch sw = new Stopwatch();
    sw.Start();
    while (e.Button == MouseButtons.Left && e.Clicks == 1 && (mouseUp == false && sw.ElapsedMilliseconds < holdButtonDuration))
        Application.DoEvents();
    if (sw.ElapsedMilliseconds < holdButtonDuration)
        btnTest_ShortClick(sender, e);
    else
        btnTest_LongClick(sender, e);
}
private void btnTest_MouseUp(object sender, MouseEventArgs e)
{
    mouseUp = true;
}