WPF Validation ErrorTemplate for Custom TextBox

Question

Branching off of this question -

When attaching a validation error template to my custom textbox like this -

<local:CustomTextBox CustomText="{Binding ViewModelProperty}" Validation.ErrorTemplate="{StaticResource errorTemplate}"/>

<ControlTemplate x:Key="errorTemplate">
    <DockPanel>
        <Border BorderBrush="Red" BorderThickness="1">
            <AdornedElementPlaceholder x:Name="controlWithError"/>
        </Border>
        <TextBlock Foreground="Red" FontSize="20" FontFamily="Segoe UI" Margin="3,0,0,0"  MouseDown="Exclamation_MouseDown"  Tag="{Binding AdornedElement.(Validation.Errors)[0].ErrorContent, ElementName=controlWithError}">!</TextBlock>
    </DockPanel>
</ControlTemplate>

If there was a validation error in the ViewModelProperty, my application was throwing an exception -

Key cannot be null.
Parameter name: key

I'm not sure why this is happening. Is there something that needs to be done in order to assign a new error template to a custom control?

UPDATE:

I've figured out that the issue is with the Tag property in the error template. If I remove the Tag, it works just fine.

Thanks

Answer 1

Alright so the way I managed to fix the issue was by removing the AdornedElement keyword and changing the error template as follows:

<local:CustomTextBox CustomText="{Binding ViewModelProperty}">
    <Validation.ErrorTemplate>
        <ControlTemplate>
            <DockPanel>
                <Border BorderBrush="Red" BorderThickness="1">
                    <AdornedElementPlaceholder x:Name="controlWithError"/>
                </Border>
                <TextBlock Foreground="Red" FontSize="20" FontFamily="Segoe UI" Margin="3,0,0,0"  MouseDown="Exclamation_MouseDown">!</TextBlock>
            </DockPanel>
        </ControlTemplate>
    </Validation.ErrorTemplate>
    <local:CustomTextBox.Style>
        <Style TargetType="{x:Type local:CustomTextBox}">
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="true">
                    <Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
                    <Setter Property="Tag" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </local:CustomTextBox.Style>
</local:CustomTextBox>

What I don't understand is why it behaves differently when using the AdornedElement keyword but works fine when binding the Tag/Tooltip using the RelativeSource. While the problem is solved, I would welcome any ideas as to why this happened.

Thanks