title: Binding multiple ComboBoxes to the same collection but filtering just one
body: <p>I have a custom control, SearchableComboBox, which inherits from ComboBox.  It simply has a textbox above the items control which filters the visible items, currently by setting the Items.Filter.</p>
  
  <p>There are multiple SearchableComboBoxes on my form all bound to the same collection.  I want them to be independently searchable, however with the way I have things wired right now, if I filter one then they all get filtered (and if one has a selection already which doesn't match the filter, its selection gets cleared).</p>
  
  <p>I've also tried adding an UnfilteredItems property to the SearchableComboBox, binding to that and then using a CollectionView created from the UnfilteredItems as the ItemsSource, then doing the filtering via the CollectionView.  This gives the exact same functionality, where all the SearchableComboBoxes are being filtered together.</p>
  
  <p>Any help with this is much appreciated, thanks in advance.</p>