title: How do I convert a ComboBox to use a bound CompositeCollection?
body: <p>I have a ComboBox that has a bound items source... I've stripped my example down to the key pieces:</p>
  
  <pre><code>&lt;UserControl x.Class="My.Application.ClientControl"
               xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
               xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"                         
               xmlns:conv="clr-namespace:My.Utilities.Converters"
               Name="ClientControl"&gt;
  
      &lt;UserControl.Resources&gt;
          &lt;ResourceDictionary&gt;
              &lt;CollectionViewSource Key="x:ClientsCollection" /&gt;
          &lt;/ResourceDictionary&gt;
  
          &lt;conv:ClientOptions x:Key="ClientOptions" /&gt;
  
      &lt;/UserControl.Resources&gt;
  
      ...
  
      &lt;ComboBox Name="Options" 
                DataContext="ClientsCollection" 
                ItemsSource="{Binding [ClientNumber], Converter={StaticResource ClientOptions}" /&gt;
  
  &lt;/UserControl&gt;
  </code></pre>
  
  <p>This works, but I now want to add a single manual item to my combobox that will trigger alternate functionality called "Other..." so I'm having to move to using the CompositeCollection... like so:</p>
  
  <pre><code>&lt;ComboBox Name="Options"
            DataContext="ClientsCollection"&gt;
      &lt;ComboBox.ItemsSource&gt;
          &lt;CompositeCollection&gt;
  
              &lt;CollectionContainer Collection="{Binding [ClientNumber], Converter={StaticResource ClientOptions} /&gt;
              &lt;ComboBoxItem&gt;Other...&lt;/ComboBoxItem&gt;
          &lt;/CompositeCollection&gt;
  &lt;/ComboBox&gt;
  </code></pre>
  
  <p>Try as I might, the bound items just won't populate when using the CompositeCollection. It only shows the manual ComboBoxItem "Other...".  If I remove that item, the list is empty. If I attach a breakpoint to the converter it doesn't catch anything, which seems to indicate that the binding isn't even attempted.</p>
  
  <p>I am obviously not understanding something about how the binding function in the CompositeCollection is happening. Can someone see an error in my XAML or explain what I'm missing?</p>