title: Combobox IsTextSearchEnabled = True validating entered text to check if it is present in Itemsource
body: <p>I have a very simple use case using <code>ComboBox</code> where <code>IsTextSearchEnabled</code>  property is <code>true</code> i.e. user can enter his text but I want to check if the entered text is present in the <code>itemsource</code>.</p>
  
  <p>Is there a simple way to  this than binding Text property and validating entered text?</p>