RadTreeView doesn't support drag selection mode. However, you can implement such with custom logic in the treeview's mouse event handlers. I prepared a small example that demonstrates a possible approach, but please keep in mind this implementation is not well tested and I cannot guarantee that it will work in all expected scenarios.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items