UsesItemPolicyAttribute Class

// The DockPanelAdornerProvider class implements an adorner
// that you can use to set the Margin property by using a 
// drag operation. The DockPanelPolicy class enables a 
// container policy for offering additional tasks and 
// adorners on the panel's children.
[UsesItemPolicy(typeof(DockPanelPolicy))]
class DockPanelAdornerProvider : AdornerProvider
{
    public DockPanelAdornerProvider() 
    {
        // The adorner is a Rectangle element.
        Rectangle r = new Rectangle();
        r.Width = 23.0;
        r.Height = 23.0;
        r.Fill = AdornerColors.GlyphFillBrush;

        // Set the rectangle's placement in the adorner panel.
        AdornerPanel.SetAdornerHorizontalAlignment(r, AdornerHorizontalAlignment.OutsideLeft);
        AdornerPanel.SetAdornerVerticalAlignment(r, AdornerVerticalAlignment.OutsideTop);

        AdornerPanel p = new AdornerPanel();
        p.Children.Add(r);

        AdornerPanel.SetTask(r, new DockPanelMarginTask());

        Adorners.Add(p);
    }
}