You can check out this article http://www.telerik.com/help/wpf/radtreeview-feautres-treeviewitem-expanding-and-collapsing-items.html
.The section Expanding and Collapsing TreeView Items Recursively
is the one you need. Please note that :
"The ExpandAll() and CollapseAll() methods work only after the first level items have been loaded. This is visible only when the treeview is bound. Therefore it should be called only in or after the Loaded event."
Therefore invoking the ExpandAll() method before the Loaded() event is fired doesn't work. I modified your example to illustrate the sequence of events. Please take a look at it and
let me know if you need more info.
the Telerik team
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