title: How to set a ComboBox's SelectedIndex to zero whenever its ItemsSource changes?
body: <p>This should be simple, but I cannot find it:
  I have two Comboboxes that are related through a Master-Detail Binding:</p>
  
  <pre><code>&lt;ComboBox Style="{StaticResource FixedSelectionCombo}"
            ItemsSource="{Binding ElementName=ControlRoot, Path=Clubs}"
            DisplayMemberPath="Name"
            SelectedItem="{Binding ElementName=ControlRoot,Path=SelectedClub}"&gt;
  &lt;/ComboBox&gt;
  &lt;ComboBox Style="{StaticResource FixedSelectionCombo}" 
            ItemsSource="{Binding ElementName=ControlRoot, Path=SelectedClub.PlayerLists}"
            DisplayMemberPath="Name"
            SelectedItem="{Binding ElementName=ControlRoot, Path=SelectedPlayerList}"&gt;
  &lt;/ComboBox&gt;
  </code></pre>
  
  <p>When I select an item in the first combobox, the second combobox gets populated with the the appropriate PlayerLists, but I want to have its first item selected automatically.</p>
  
  <p>This is easy to do in code behind, but I want to achieve this through a Style that I can put in a ResourceDictionary. I tried:</p>
  
  <pre><code>  &lt;Style x:Key="FixedSelectionCombo" TargetType="ComboBox" BasedOn="{StaticResource {x:Type ComboBox}}"&gt;
          &lt;Setter Property="SelectedIndex" Value="0"/&gt;
      &lt;/Style&gt;
  </code></pre>
  
  <p>But this only works the first time, not after I make a new selection in the first combobox.</p>
  
  <p>How can it be done?</p>