title: WPF ObservableCollection: How to add a blank line in one form's combobox, but not actually affect the ObservableCollection?
body: <p>I have a static ObservableCollection in a Data Repository class. I use it to populate a combobox on one of my forms (which needs to be able to include a blank line which represents NULL).</p>
  
  <p>I use the same ObservableCollection to populate a DataGrid, so I don't want the blank item in the actual ObservableCollection. How do I actually do this?</p>
  
  <p>Oh, and the reason I want to do this is so that if I have both forms open and I delete an item from the ObservableCollection it should reflect that in both of the lists.</p>