title: WPF datagrid grouping
body: <p>I'm grouping my grid like this:</p>
  
  <pre><code>ListCollectionView collection = new ListCollectionView(_ActionCollection);
              collection.GroupDescriptions.Add(new PropertyGroupDescription("InvoiceNumber"));
              dataGrid1.ItemsSource = collection;
  </code></pre>
  
  <p>Now I want to get the sum of the items which are grouped.</p>
  
  <p>For example:</p>
  
  <p>If grouped ivnoice number 231 and 245 contain both 3 rows, how can I get the sum for the example id?</p>