wpf error template - red box still visible on collapse of an expander

Question

I'm doing some validation on the DataSource of TextBox that's within an Expander and have found that once a validation error has been triggered, if I collapse the Expander, the red box stays where the TextBox would have been.

<Expander Header="Blah Blah Blah">
  <TextBox Name="TextBox"
           Validation.ErrorTemplate="{DynamicResource TextBoxErrorTemplate}"
           Text="{Binding Path=Blah,
                          UpdateSourceTrigger=PropertyChanged,
                          ValidatesOnDataErrors=True}" />
</Expander>

I've tried to get round this by binding the visibility of the Error Template to the Expander, however I think there's something wrong with the binding.

<local:NotVisibleConverter x:Key="NotVisibleConverter" />

<ControlTemplate x:Key="TextBoxErrorTemplate">
  <DockPanel>
    <Border BorderBrush="Red" BorderThickness="2" 
            Visibility="{Binding Path=IsExpanded, 
                                 Converter={StaticResource NotVisibleConverter}, 
                                 RelativeSource={RelativeSource AncestorType=Expander}}" >
      <AdornedElementPlaceholder Name="MyAdorner" />
    </Border>
  </DockPanel>
  <ControlTemplate.Triggers>
    <Trigger Property="Validation.HasError" Value="true">
        <Setter Property="ToolTip"
                Value="{Binding RelativeSource={RelativeSource Self}, 
                                Path=(Validation.Errors)[0].ErrorContent}"/>
    </Trigger>
  </ControlTemplate.Triggers>
</ControlTemplate>

I guess I've gone wrong with my binding, can someone put me back on track please? Alternatively does anyone know another solution to the ErrorTemplate still being visible on the collapse of an Expander?

Answer 1

Rather than doing any binding, you could place an AdornerDecorator around the elements inside of your expander. You see, the validation error template is placed on the adorner layer that way it shows up on top of everything else. That's ultimately what your problem is. Even though your text box is not visible because the expander is closed, the error template is still on the adorner layer.

I believe you can fix this with the following xaml:

<Expander Header="Blah Blah Blah">
   <AdornerDecorator>
      <TextBox Name="TextBox"
               Validation.ErrorTemplate="{DynamicResource TextBoxErrorTemplate}"
               Text="{Binding Path=Blah,
                              UpdateSourceTrigger=PropertyChanged,
                              ValidatesOnDataErrors=True}" />
   </AdornerDecorator>
</Expander>

This creates an adorner layer specifically for within the expander. When the expander is closed the AdornerDecorator also gets hidden and so should everything on it.