title: Why is Converter not called when Collection changes?
body: <p>I made a <code>CollectionToStringConverter</code> which can convert any <code>IList</code> into a comma-delimited string (e.g. "Item1, Item2, Item3").</p>
  
  <p>I use it like this:</p>
  
  <pre><code>&lt;TextBlock Text="{Binding Items, 
                    Converter={StaticResource CollectionToStringConverter}}" /&gt;
  </code></pre>
  
  <p>The above works, but only once when I load the UI. <code>Items</code> is an <code>ObservableCollection</code>. The text block does not update, and the converter does not get called when I add or remove from <code>Items</code>. </p>
  
  <p>Any idea what's missing to make this work?</p>