ControlTemplate for adding UIElements to original control

Question

I want to add an exclamation mark Image to the left of the built-in TextBox and make it visible whenever the TextBox Validation.HasError attached property is true, otherwise hide it.

How can I use ControlTemplate to add the Image without having re-bind all the TextBox properties?

<StackPanel>
    <StackPanel.Resources>
        <ControlTemplate x:Key="TextBoxWithIndicator" TargetType="{x:Type TextBox}">
            <StackPanel Orientation="Horizontal">
                <!-- Re-bind {Binding Path=Property}, including some that I may miss -->
                <TextBox Text="{TemplateBinding Text}" Width="{TemplateBinding Width}" Height="{TemplateBinding Height}"/>
                <Image Source="resources/exclaim.png" Visibility="{TemplateBinding Validation.HasError}"/>
            </StackPanel>
        </ControlTemplate>
    </StackPanel.Resources>
    <TextBox Template="{StaticResource TextBoxWithIndicator}" Width="120">Happy Go Lucky</TextBox>
</StackPanel>

Note The preceding block of code represents my futile effort in WPF so far. It is probably also wrong on several counts, e.g. probably need a ValueConverter for Visibility <--> Validation.HasError; Setting Width="120" on TextBox seems to adjust the StackPanel width instead of TextBox width despite the TemplateBinding, etc.

Answer 1

I would suggest looking into Adorners. These are special FrameworkElements that are rendered in a special Adorner Layer on top of visual elements, and are intended to provide visual cues to the user.

The above link provides a summary of Adorners as well as an example of a Custom Adorner.