title: Set index on ComboBox.ItemsSource changed
body: <p>I'm using Bindings to set the ItemsSource of a combobox.
  How can I set the index of my combobox to 0 without an event handler or will I have to use an event handler?</p>
  
  <p>SelectedIndex in WPF gets ignored since SelectedValue is already assigned to a binding.</p>
  
  <pre><code>&lt;ComboBox SelectedValue="{Binding Class}" 
            ItemsSource="{Binding Source={StaticResource classList}}"
            SelectedIndex="0"/&gt;
  &lt;ComboBox SelectedValue="{Binding ItemSubClass}" 
            ItemsSource="{Binding Class.AvailableSubClasses}" 
            SelectedIndex="0"/&gt;
  </code></pre>
  
  <p>Selected value on cb2 is null when cb1.value changes. And cb1.value is null on load.</p>