How can I trigger this Error Template?

Question

Below is a template that works from a binding perspective, but the error template doesn't show, and without an AdornedElementPlaceholder the result looks a bit garish.

My view models implement IDataErrorInfo, and normally I would trigger the error template by having ValidatesOnError=True as part of my binding. This particular view model is display only, so the IDataErrorInfo indexer is never invoked. I do have a number of useful properties related to validation though, including a boolean IsValid property as well as IDataErrorInfo.Error, both of which properly respond to the view model being invalid.

Should I translate the error to a ValidationResult and trigger it that way? Or is there something easier?

Cheers,
Berryl

current template

<!-- FooterViewModel DataTemplate -->
<DataTemplate DataType="{x:Type model:FooterViewModel}">

    <Label x:Name="lblTotalTime"
        Style="{StaticResource FooterStyle}" 
        Content="{Binding TotalTime, Converter={StaticResource TotalAmountConv}}" >
        <Label.ToolTip>
            <TextBlock Text="{Binding FeedbackMessage}" ></TextBlock>
        </Label.ToolTip>
        <Validation.ErrorTemplate>
            <ControlTemplate>
                <DockPanel LastChildFill="True">
                    <TextBlock DockPanel.Dock="Right" Text=" *" 
                               Foreground="Red" 
                               FontWeight="Bold" FontSize="16" 
                               />
                    <Border BorderBrush="Red"  BorderThickness="1">
                        <AdornedElementPlaceholder Name="placeholder"></AdornedElementPlaceholder>
                    </Border>
                </DockPanel>
            </ControlTemplate>
        </Validation.ErrorTemplate>
    </Label>

    <DataTemplate.Triggers>
        <DataTrigger Binding="{Binding IsValid}" Value="False">
            <Setter TargetName="lblTotalTime" Property="Control.BorderBrush" Value="Red"/>
            <Setter TargetName="lblTotalTime" Property="Control.BorderThickness" Value="1"/>
            <Setter TargetName="lblTotalTime" Property="Control.Background" Value="LightYellow"/>
        </DataTrigger>
    </DataTemplate.Triggers>

</DataTemplate>

UPDATE

Ok, I am getting IDataErrorInfo to kick in just by changing my binding to include ValidatesOnErrors, BUT the error template still does not show up.

Here is the binding

    <ItemsControl 
        ItemsSource="{Binding Path=FooterViewModels, Mode=OneWay, ValidatesOnDataErrors=True}">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal"/>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>

Answer 1

By default, the validation is only run when the Source of the binding is updated. In your ItemsControl.ItemsSource binding the Sources is your FooterViewsModels, which obviously will never be updated (because you have Mode=OneWay).

You can use the DataErrorValidationRule.ValidatesOnTargetUpdated to run the validation when the target is updated as well. The link gives an example.

Keep in mind that the Binding.ValidatesOnDataErrors property is is just a short cut for adding an instance of DataErrorValidationRule to the Binding.ValidationRules collection.

Finally, the control that the binding is defined one will have the Validation.Errors. In your case, that is the ItemsControl, not the items inside it. So, I believe you need to add the DataErrorValidationRule to your Label.Content binding. Or you need to define your ErrorTemplate on the ItemsControl, depending on what you are going for.