DispatcherTimer _doubleClickTimer;

public Page()
{
    InitializeComponent();
    _doubleClickTimer = new DispatcherTimer();
    _doubleClickTimer.Interval = new TimeSpan(0, 0, 0, 0, 200);
    _doubleClickTimer.Tick += new EventHandler(DoubleClick_Timer);
    this.MouseLeftButtonDown += new MouseButtonEventHandler(Page_MouseLeftButtonDown);
}

// too much time has passed for it to be a double click.
void DoubleClick_Timer(object sender, EventArgs e)
{
    _doubleClickTimer.Stop();
}

void Page_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    if (_doubleClickTimer.IsEnabled)
    {
        // a double click has occured
        _doubleClickTimer.Stop();
    }
    else
    {
        _doubleClickTimer.Start();
    }
}