To drag usercontrol as popup

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WPF

Hi
How to drag popup window on mouse move. I used wpf user control as popup view.

The below code works but that position of popup is not correct.

Please help me.

private TranslateTransform transform = new TranslateTransform();
private void UserControl_MouseMove(object sender, MouseEventArgs e)
{
     if (isInDrag)
     {
           var element = sender as FrameworkElement;
           currentPoint = e.GetPosition(null);
           transform.X += currentPoint.X - anchorPoint.X;
           transform.Y += (currentPoint.Y - anchorPoint.Y);
           this.RenderTransform = transform;
           anchorPoint = currentPoint;
      }
}
private void PopUp_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
      var element = sender as FrameworkElement;
      anchorPoint = e.GetPosition(null);
      element.CaptureMouse();
      isInDrag = true;
      e.Handled = true;
}
private void PopUp_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
      if (isInDrag)
      {
          var element = sender as FrameworkElement;
          element.ReleaseMouseCapture();
          isInDrag = false;
          e.Handled = true;
      }
}