Itemsproperty has been set and the data can be filtered to provide possible matches in the drop-down.
public void PopulateComplete()
Call this method when you are providing custom population of the drop-down portion of the AutoCompleteBox, to signal the control that you are done with the population process. Typically, you use PopulateComplete when the population process is a long-running process and you want to cancel built-in filtering of the ItemsSource items. In this case, you can handle the Populated event and set PopulatingEventArgs.Cancel to true. When the long-running process has completed you call PopulateComplete to indicate the drop-down is populated.