title: How do you add a generic item to a ComboBox bound to a collection in WPF
body: <p>I have a ComboBox in a WPF application that is bound to an ObservableCollection of Department objects in a C# ViewModel class. I want to use the combo box to filter another collection by department (And indeed it works for that now) The problem is that I want to add an additional option "All" to the top of the list. Is there a correct way to do this. Making a fake department feels wrong in so many ways.</p>
  
  <p>The ComboBox</p>
  
  <pre><code>&lt;ComboBox ItemsSource="{Binding Path=Departments}" 
            SelectedValue="{Binding Path=DepartmentToShow , Mode=TwoWay}" /&gt;
  </code></pre>