title: How to filter a collection when binding it to ItemsSource?
body: <p>I created a treeview which modelizes directories and files on my hard drive. Each treeviewItem has a checkbox, binded to a isSelected property. What I'd like to achieve is display for each parent node the count of selected files on the total files count (10 / 12 10 files on twelve total selected).</p>
  
  <p>Is there a way to do a binding with where property is ...?</p>
  
  <pre><code>&lt;ContentPresenter Content="{Binding MyItems.Count where MyItems.IsSelected, Mode=OneTime}"
                    Margin="2,0" /&gt;
  </code></pre>