How can I highlight the cell which contains the error? - wpf

Question

I have a DataGrid with four columns for which I have defined a style as well as a triggered style for the case the user enters an invalid value.
<Style TargetType="{x:Type DataGridRow}">
<Setter Property="FontSize" Value="16"/>
<Setter Property="FontFamily" Value="ArialMT"/>
<Setter Property="Height" Value="24"/>
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="ValidationErrorTemplate">
<Setter.Value>
<ControlTemplate>
<Grid>
<Ellipse Width="12" Height="12" Fill="Red" Stroke="Black" StrokeThickness="0.5"/>
<TextBlock FontWeight="Bold" Padding="4,0,0,0" Margin="0" VerticalAlignment="Top" Foreground="White" Text="!" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="BorderBrush" Value="Red"/>
<Setter Property="IsEnabled" Value="True" />
<Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>
This works well and the complete DataGridRow is marked as faulty because I used this in the XAML:
<DataGrid.RowValidationRules>
<local:CycleValidationRule ValidationStep="UpdatedValue" />
</DataGrid.RowValidationRules>
Now I want to highlight the DataGridCell with the invalid value explictely additionally (setting the background colour). Hence, I defined another style:
<Style x:Key="cycleErrStyle" TargetType="{x:Type TextBlock}">
<Setter Property="Background" Value="Magenta"/>
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true" >
<Setter Property="Background" Value="Red" />
<Setter Property="Foreground" Value="White" />
</Trigger>
</Style.Triggers>
</Style>
but this doesn't work.
When I set <Trigger Property="Validation.HasError" Value="false" > to false, the style affects. It seems as if the Validation.HasError property has been reset after validation for the grid's row.
In the XAML I defined this:
<DataGridTextColumn x:Name="TagCycle" Header="Cycle" Binding="{Binding Mode=TwoWay, Path=RawTag.Cycle, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}"
ElementStyle="{StaticResource ResourceKey=cycleErrStyle}" />
How can I highlight the invalid cell additionally to marking the row as faulty?

Answer 1

You could use a DataTrigger that binds to the Validation.HasError attached property of the parent DataGridRow:
<Style x:Key="cycleErrStyle" TargetType="{x:Type TextBlock}">
<Setter Property="Background" Value="Magenta"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=(Validation.HasError), RelativeSource={RelativeSource AncestorType=DataGridRow}}" Value="true" >
<Setter Property="Background" Value="Red" />
<Setter Property="Foreground" Value="White" />
</DataTrigger>
</Style.Triggers>
</Style>