You can achieve the desired behavior by getting the WatermarkTextBox from the AutoCompleteBox template and setting its SelectionOnFocus property to SelectAll. However you would need to do in the Loaded event handler of the AutoCompleteBox. So if you have the following AutoCompleteBox:
The Loaded handler should look like shown below:
sender, RoutedEventArgs e)
var textBox =
textBox.SelectionOnFocus = SelectionOnFocus.SelectAll;
This way the text will be highlighted whenever the AutoCompleteBox got the focus.
Hope this helps.
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