title: TextBox bound to ObservableCollection not updating
body: <p>I have a textbox bound to an observablecollection, and when I update the element (via a drag and drop which fires an event which is handled in the view file) the textbox does not update its value. However, the data is added to the observable collection on the drop and if I do a refresh of the data (by actually selecting a different item in a listbox and switching back to the current record) the data appears.</p>
  
  <p>I have read: <a href="http://updatecontrols.net/doc/tips/common_mistakes_observablecollection" rel="nofollow">http://updatecontrols.net/doc/tips/common_mistakes_observablecollection</a> and no I don't believe I'm overwriting the collection!</p>
  
  <pre><code>&lt;StackPanel&gt;
      &lt;TextBox Text="{Binding Path=ImageGalleryFilenames, Converter={StaticResource ListToStringWithPipeConverter}}" Height="41" TextWrapping="Wrap" VerticalAlignment="Top"/&gt;
      &lt;Button Height="25" Margin="0 2" AllowDrop="True" Drop="HandleGalleryImagesDrop"&gt;
          &lt;TextBlock Text="Drop Image Files Here"&gt;&lt;/TextBlock&gt;
      &lt;/Button&gt;
  &lt;/StackPanel&gt;
  </code></pre>
  
  <p>This is my event code for handling the drop in the view file for the user control.</p>
  
  <pre><code>    private void HandleGalleryImagesDrop(object sender, DragEventArgs e)
      {
          if (e.Data.GetDataPresent(DataFormats.FileDrop))
          {
              var filenames = (string[])e.Data.GetData(DataFormats.FileDrop);
  
              foreach (var fn in filenames)
              {
                  this.vm.CurrentSelectedProduct.ImageGalleryFilenames.Add(fn);
              }
          }
      }
  </code></pre>
  
  <p>Shouldn't the fact that I'm adding to the collection be enough to update the textbox that's bound to the observablecollection or am I missing something blindingly obvious?</p>