Question

I have defined following in a ResourceDictionary:

<DockPanel x:Key="errorDisplay" LastChildFill="False">
        <Border Background="Red" DockPanel.Dock="Top">
            <TextBlock x:Name="errorTextBlock" />
        </Border>
        <Canvas DockPanel.Dock="Top" Height="15">
            <Polygon 
                Points="{Binding ElementName=errorDisplay, Path=ActualWidth, Converter={StaticResource PointsToStringArrayConverter},Mode=OneWay}"
                Fill="Red" Stroke="Black" StrokeThickness="1" />                
        </Canvas>
</DockPanel>

This resource is added to an adorner layer in code behind on loading of a usercontrol.

However, I get a binding error

(Cannot find source for binding with reference
‘ElementName=errorDisplay’)

. I am aware of namescopes, but surely the above should work as it all takes place within the single composite control, with the same namescope applying?

EDIT (Using RelativeSource/AncestorType instead of elementname does not seem to work either.) Does work! But actualwidth is Nil

EDIT: using ElementName=errorTextBlock also results in same binding error!

Answer 1

You haven’t named the DockPanel you have provided a resource key.

Try <DockPanel x:Key="errorDisplay" x:Name="errorDisplay" LastChildFill="False">

However as you say, this doesn’t work when tested.

A relative source binding seems to work in this cut down version of your code though:

This is in a resource element

<DockPanel x:Key="errorDisplay" LastChildFill="False">
    <Border Background="Red" DockPanel.Dock="Top">
        <TextBlock x:Name="errorTextBlock" 
         Text="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type DockPanel}}, Path=ActualHeight, Mode=OneWay}" />
    </Border>
</DockPanel>

and I’m using it like this:

<ContentControl Content="{StaticResource errorDisplay}"/>