title: WPF Datagrid ContextMenu SelectedItem
body: <p>I have the following problem.
  I have a DataGrid with a ContextMenu. But when the Command is triggered the SelectedItem is always null. <br>Any recommendations?</p>
  
  <p>Thats my ContextMenu: </p>
  
  <pre><code>&lt;DataGrid.ContextMenu&gt;
    &lt;ContextMenu&gt;
    &lt;MenuItem Command="{Binding OpenFileCommand}"
             CommandParameter="{Binding Path=SelectedItem,
             RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"
             Header="open file" /&gt;
  </code></pre>
  
  <p>
   </p>
  
  <p>i also tried:
  <code>CommandParameter="{Binding ElementName=nameOfGrid, Path=SelectedItem}"</code></p>