Occurs when the
AutoCompleteBoxis populating the drop-down with possible matches based on the
public event EventHandler<PopulatingEventArgs> Populating
If the event is canceled, by setting the PopulatingEventArgs.Cancel property to true, the AutoCompleteBox will not automatically populate the selection adapter contained in the drop-down. In this case, if you want possible matches to appear, you must provide the logic for populating the selection adapter.