The simplest Link is implemented with a DataTemplate that consists of a Shape that is the stroke (a line) connecting one node with another. In WPF that is normally an instance of LinkShape; in Silverlight that is normally an instance of System.Windows.Shapes.Path. Such a link cannot have any arrowheads or text labels or other decorations. Furthermore you cannot connect any link to or from the link itself, nor can any link labels be selectable or manipulable as separate objects.

However, most Diagram.LinkTemplates are implemented as DataTemplates consisting of LinkPanels. The principal child element of a LinkPanel is normally a LinkShape (WPF) or Path (Silverlight) that has the x:Name of "Path" and that has the attached property IsLinkShapeProperty set to true. The panel arranges the other child elements along the Link's Route according to various attached properties.

At the current time a LinkPanel or LinkShape/Path must be the root visual element of a Link.

Here is a simple DataTemplate for a Link that has an arrowhead:

              <DataTemplate x:Key="ExampleLinkTemplate">
                <go:LinkPanel go:Link.SelectionElementName="Path" go:Link.SelectionAdorned="True" >
                  <go:LinkShape x:Name="Path" go:LinkPanel.IsLinkShape="True" Stroke="Black" StrokeThickness="1" />
                  <Polygon Fill="Black" Points="8 4  0 8  2 4  0 0" go:LinkPanel.Alignment="1 0.5" go:LinkPanel.Index="-1" go:LinkPanel.Orientation="Along" />
                </go:LinkPanel>
              </DataTemplate>
            

System.Object
   System.Windows.DependencyObject
      System.Windows.UIElement
         System.Windows.FrameworkElement
            System.Windows.Controls.Panel
               Northwoods.GoXam.LinkPanel

Reference