The RadAutoCompleteBox exposes almost all of the TextBox events. Additionally, it provides the following events:
SelectionChanged: Occurs when an item of the RadAutoCompleteBox is selected. The type of the passed arguments is SelectionChangedEventArgs. You can use the readonly AddedItems and RemovedItems properties to see what item(s) were selected or deselected.
SearchTextChanged: Occurs when the text in the text box portion of the RadAutoCompleteBox changes. This event should be used instead of the TextChanged event. The type of the passed arguments is EventArgs.
Populating: Occurs when RadAutoCompleteBox starts populating its popup with items. The type of the passed arguments is CancelRoutedEventArgs meaning that you can cancel the event based on your custom logic by setting the Cancel property of the arguments to True.
Populated: Occurs after RadAutoCompleteBox populates its popup with items. The type of the passed arguments is EventArgs.