title: WPF datagrid: disable editing in certain rows
body: <p>I have something like orders table bound to a <code>DataGrid</code>. 
  I want to disable editing on rows, where order date is expired (deadline date > current date): show them greyed and ban entering into edit mode.</p>
  
  <p>Here is what I have:</p>
  
  <pre><code>&lt;Style TargetType="{x:Type WPFToolkit:DataGridRow}"&gt;
      &lt;Style.Triggers&gt;
          &lt;MultiTrigger&gt;
              &lt;MultiTrigger.Conditions&gt;
                  &lt;!-- ??? order_date &gt; current_date --&gt;
                  &lt;!-- some other condition - already works --&gt;
              &lt;/MultiTrigger.Conditions&gt;
              &lt;Setter Property="IsEnabled" Value="False"/&gt;
              &lt;Setter Property="Foreground" Value="LightGray"/&gt;
          &lt;/MultiTrigger&gt;
      &lt;/Style.Triggers&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>The order date is a template column defined as follows:</p>
  
  <pre><code>&lt;WPFToolkit:DataGridTemplateColumn SortMemberPath="order_date" Header="Deadline" &gt;
      &lt;WPFToolkit:DataGridTemplateColumn.CellTemplate&gt;
          &lt;DataTemplate&gt;
              &lt;TextBlock Text="{Binding Path=order_date,  
                  ConverterCulture='ru-RU', StringFormat='{}{0:d}'}" /&gt;
          &lt;/DataTemplate&gt;
      &lt;/WPFToolkit:DataGridTemplateColumn.CellTemplate&gt;
      &lt;WPFToolkit:DataGridTemplateColumn.CellEditingTemplate&gt;
          &lt;DataTemplate&gt;
              &lt;WPFToolkit:DatePicker SelectedDate="{Binding Path=order_date}" /&gt;
          &lt;/DataTemplate&gt;
      &lt;/WPFToolkit:DataGridTemplateColumn.CellEditingTemplate&gt;
  &lt;/WPFToolkit:DataGridTemplateColumn&gt;
  </code></pre>
  
  <p>Is there a way to disable editing mode in rows where order date has expired?</p>