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The RadAutoCompleteBox control does not support an ItemsSource of type DataTable - it expects its data source to be of type IEnumerable.
However, the control provides a way to customize its filtering behavior ( through the FilteringBehavior property), so actually you can take the items and manually filter them from any datasource you need.
I have attached a very simple project demonstrating the approach. Note the CustomFilteringBehavior class - the FindMatchingItems method should return a list of the filtered items.
Hope this helps.
the Telerik team