title: change background of DataGrid CheckBox Column for validation errors
body: <p>(Visual Studio 2010)  I'm trying to perform validation on a <code>CheckBoxColumn</code> in a WPF datagrid.  I've set up a validation rule, and if it fails I want the cell background to turn red and have tooltip:</p>
  
  <pre><code> &lt;DataGridCheckBoxColumn Header="Checked" &gt;
                  &lt;DataGridCheckBoxColumn.Binding&gt;
                      &lt;Binding Path="CheckProperty" Mode="TwoWay" ValidatesOnDataErrors="True" UpdateSourceTrigger="PropertyChanged" &gt;
                          &lt;Binding.ValidationRules&gt;
                              &lt;local:CheckValidationRule ValidationStep="UpdatedValue" /&gt;
                          &lt;/Binding.ValidationRules&gt;
                      &lt;/Binding&gt;
                  &lt;/DataGridCheckBoxColumn.Binding&gt;
                  &lt;DataGridCheckBoxColumn.CellStyle&gt;
                      &lt;Style TargetType="DataGridCell"&gt;
                          &lt;Style.Triggers&gt;
                              &lt;Trigger Property="Validation.HasError" Value="True"&gt;
                                  &lt;Setter Property="Background" Value="Red"/&gt;
                                  &lt;Setter Property="ToolTipService.ToolTip" 
                                Value="{Binding RelativeSource={RelativeSource Self},
                                  Path=(Validation.Errors).CurrentItem.ErrorContent}"/&gt;
                              &lt;/Trigger&gt;
                          &lt;/Style.Triggers&gt;
                      &lt;/Style&gt;
                  &lt;/DataGridCheckBoxColumn.CellStyle&gt;
              &lt;/DataGridCheckBoxColumn&gt;
  </code></pre>
  
  <p>But when the validation fails, my cell style is completely ignored.  I'm actually used to this (but I never figured <em>why</em> it was ignored), so then I tried to use a <code>TemplateColumn</code> with a <code>Checkbox</code>, and there I can change the <code>CheckBox</code> border background, but still haven't figured out how to change the background color of the entire cell itself.</p>