title: ColumnHeader arrows not reflected when sorting a DataGrid in XAML
body: <p>I have a DataGrid with some sorting defined in XAML like so:</p>
  
  <pre><code>&lt;CollectionViewSource x:Key="DefaultSort" Source="{Binding SearchResults}"&gt;
      &lt;CollectionViewSource.SortDescriptions&gt;
          &lt;scm:SortDescription PropertyName="ExternalOrgNo" Direction="Ascending"/&gt;
          &lt;scm:SortDescription PropertyName="ExternalBranchNumber" Direction="Ascending"/&gt;
      &lt;/CollectionViewSource.SortDescriptions&gt;
  &lt;/CollectionViewSource&gt;
  </code></pre>
  
  <p>The sorting is properly applied to the DataGrid but there is no indication of the sorting on the grid.</p>
  
  <p>Looking at the view in the code behind I see the SortDescriptions in the collection and I've tried refreshing the view but it did not work.</p>
  
  <p>How can I have the ColumnHeader arrows properly reflect the status of the view's SortDescription collection initially?</p>
  
  <p>UPDATE:  I found an answer.  I added the SortDirection to the DataGridTextColumn in the DataGrid.  This added the ColumnHeader arrows.</p>
  
  <pre><code>&lt;DataGridTextColumn Header="Ext Firm #" Binding="{Binding ExternalOrgNo}" DisplayIndex="4" SortDirection="Ascending" Visibility="Visible" /&gt;
  &lt;DataGridTextColumn Header="Ext Branch #" Binding="{Binding ExternalBranchNumber}" DisplayIndex="5" SortDirection="Ascending" Visibility="Visible" /&gt;
  </code></pre>