Because there are no data objects for links in models such as GraphModel and TreeModel, we keep track of links by instead remembering the two nodes that they connect. This also becomes the value for the PartManager.PartBinding.Data property for links, so that link data templates can have access to the node data at either end of the link relationship. This class is not used when the model is Northwoods.GoXam.Model.ILinksModel.