title: Cancel collection changed event on an observable collection
body: <p>How can i cancel the collection changed event on an observable collection?
  When my collection changes it invokes methods on a third party dll.These methods may or may not fail. </p>
  
  <p>If they fail, i want dont want the item to be added to or removed from the collection. Looking at the name, it looks like the collection changed event is fired after something has been added or deleted, but how could i achieve my functionality?</p>