Drag WPF Popup control

<Popup x:Name="pop" IsOpen="True" Height="200" Placement="AbsolutePoint"  Width="200">
   <Rectangle Stretch="Fill" Fill="Red"/>
</Popup>

pop.MouseMove += new MouseEventHandler(pop_MouseMove);

   void pop_MouseMove(object sender, MouseEventArgs e)
    {
        if (e.LeftButton == MouseButtonState.Pressed)
        {
            pop.PlacementRectangle = new Rect(new Point(e.GetPosition(this).X,
                e.GetPosition(this).Y),new Point(200,200));

        }
    }

Private Point startPoint;

 private void Window_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {

        startPoint = e.GetPosition(null);
    }
private void Window_MouseMove(object sender, MouseEventArgs e)
    {
        if (e.LeftButton == MouseButtonState.Pressed)
        {
            Point relative = e.GetPosition(null);
            Point AbsolutePos = new Point(relative.X + this.Left, relative.Y + this.Top);
            this.Top = AbsolutePos.Y - startPoint.Y;
            this.Left = AbsolutePos.X - startPoint.X;
        }
    }