Adding an ErrorTemplate to a WPF User Control disables TextBox input

Question

I have a very basic custom control consisting of a Label and a Textbox. I've used my control for sometime without any issues.

I've now come to styling my application and have my style inside a XAML file containing just a ResourceDictionary. I have the following for my UserControl:

<Style TargetType="local:LabelEdit">
    <Setter Property="Background" Value="{StaticResource BackgroundBrush}" />
    <Setter Property="Foreground" Value="{StaticResource ForegroundBrush}" />
    <Setter Property="BorderBrush" Value="{StaticResource BorderBrush}" />

    <Setter Property="Validation.ErrorTemplate">
        <Setter.Value>
            <ControlTemplate>
                <StackPanel>
                    <AdornedElementPlaceholder />
                    <Image Source="/AJSoft.Controls;component/Resources/Icons/cross.ico" />
                </StackPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>

    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="True">
            <Setter Property="Foreground" Value="{StaticResource ErrorForegroundBrush}" />
            <Setter Property="Background" Value="{StaticResource ErrorBackgroundBrush}" />
            <Setter Property="BorderBrush" Value="{StaticResource ErrorBorderBrush}" />
        </Trigger>
    </Style.Triggers>
</Style>

Everything works absolutely fine if I comment out the Setter for Validation.ErrorTemplate. If the ErrorTemplate is left intact, the cross shows (I haven't sorted out placement yet, but that can come later...), but the Textbox component of my UserControl does not show the caret or accept keyboard input. The rest of the controls in my Window work as expected.

Here are some screenies where I've deliberately put in some erroneous text to show how it looks.

The same problem happens even if I change that huge image to be a textblock with a small red "!" - the image is just for effect for now.

What am I doing that's causing the problem? I'm new to Validation in WPF...

EDIT: The image shown (big red cross) is just one example of what I've done. Even if I use a small image shown alongside the UserControl, I still get the same effect.

Answer 1

If you were to look at how error templates usually work, you'd see they apply to a single control. Part of the issue you have here is you've got a label and textbox in one parent user control. If you then apply an error template at the usercontrol level, it's on everything in that. Label, textbox, everything in your usercontrol.

The next thing to consider is how your error template ends up visible on top of everything. This happens because your error template generates UI in the adorner layer. That's top of everything ( in the window ). Add these together and you got a big image on top of the content of your usercontrol.

At risk of over simplifying:

You put a top on your box and you can't now get at what's in that box.

There are several ways you could "fix" this but they all involve some design change or compromise. Maybe a big X on top of your input control isn't a good idea. You could kind of make it work.

You could make your image IsHitTestVisible="False". It'll be visually in the way but you can then likely click on the textbox and type. Just maybe not see everything. Probably not ideal.

You could show your cross next to the textbox using a datatrigger rather than error template. Add an image to your usercontrol so you have label, textbox, CrossImage. Add a style to that with a setter makes it visible collapsed by default. Use a trigger in that style to show the CrossImage when the control has errors.

 <Style.Triggers>
    <Trigger Property="Validation.HasError" Value="true">
        <Setter ... />
    </Trigger>
 </Style.Triggers>

You may well find it simplest to use the tag on the usercontrol and set that to visible/collapsed. Bind visibility of the image to that.