Question

I want to add some elements to a TextBox by using a Template with the extra elements and the original TextBox inserted in the right spot. I’m trying to use the AdornedElementPlaceholder just like you would do when making a Validation.ErrorTemplate But the AdornedElement is not showing up. I have simplified the example as much as possible:

<TextBox Text="Border me with my Template">
     <TextBox.Template>
      <ControlTemplate TargetType="{x:Type TextBox}">
       <Border BorderBrush="Green" BorderThickness="1">
        <AdornedElementPlaceholder/>
       </Border>
      </ControlTemplate>
     </TextBox.Template>
    </TextBox>  

The result is just a green box around the space that should be my textbox!

Answer 1

Unfortunately, that’s just not how it works. However, it’s not much more difficult than that. If you want to add a green border around a text box by changing the control template, it’s going to look a bit more like this:

<ControlTemplate TargetType="TextBox">
    <Border x:Name="Border"
            CornerRadius="2"
            Background="{TemplateBinding Background}"
            BorderBrush="Green"
            BorderThickness="1"
            SnapsToDevicePixels="True">
        <ScrollViewer x:Name="PART_ContentHost"/>
    </Border>
</ControlTemplate>

The important part in there is the ScrollViewer named PART_ContentHost. The TextBox looks for that guy when the template is applied and uses it to host the text, caret, etc. What I think you’re missing is that when you override the Template for an element, you’re overriding the entire control template. It’s unfortunate that you can’t just change bits and pieces, but that’s the truth of the matter.

Of course, if you want to maintain the original look and feel of the TextBox, such that it still looks like a Win7 TextBox for instance, you’d need to do a bit more in the ControlTemplate.

For what it’s worth, it looks like the template you were trying to apply would work if you’re talking about using an Adorner. It’s similar to how the validation templates work in WPF, but that’s a whole-nother story.

Oh, and for a much simpler way to change the border to green, you should be able to just set the BorderBrush property on the TextBox itself. Of course, I really don’t know exactly what you’re going for.

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HTH
Dusty