ItemPrepared event is fired when the item container is created and ready for use. Which is just about before displaying the item. So the event may not be fired when item is added to the collection.
You can check the example for "Programatic Expand" in "Controls\NAVIGATION\TreeView" section under the following link http://demos.telerik.com/wpf/. It represents an alternative way in which a particular item can be expanded. The example uses data bound tree view.
If your tree is data bound then you can customize which field from your data will be used by the ExpandItemByPath function. This can be done via TextSearch.Text attached property of the tree view. Following code snippet shows how you can set the ExpandItemByPath to use the Name property of the Entity business object.
If your tree view is not data bound, then you can try expanding the parent item containers all the way from root to your desired item. This approach hides many difficulties because the item containers may not be created when you need them.
Hope this helps. Please let us know if you need more help.
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