RelativeSource FindAncestor Differences in WPF 4 & Silverlight 5

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WPF

Silverlight

<TextBox HorizontalAlignment="Left" Height="23" TextWrapping="Wrap" Text="TextBox1" VerticalAlignment="Top" Width="120">
                <interactivity:Interaction.Triggers>
                    <interactivity:EventTrigger EventName="GotFocus">
                        <interactivity:InvokeCommandAction Command="{Binding GotFocusCommand}" CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=TextBox}}" />
                    </interactivity:EventTrigger>
                </interactivity:Interaction.Triggers>
            </TextBox>

In WPF 4, the CommandParameter is correctly set to the focused Textbox. But in SL5, the CommandParameter is set to null.

But the following works in both WPF 4 & SL5:

<TextBox HorizontalAlignment="Left" Height="23" TextWrapping="Wrap" Text="TextBox1" VerticalAlignment="Top" Width="120">
    <interactivity:Interaction.Triggers>
        <interactivity:EventTrigger EventName="GotFocus">
            <interactivity:InvokeCommandAction Command="{Binding GotFocusCommand}" CommandParameter="{Binding Path=Children[0], RelativeSource={RelativeSource FindAncestor, AncestorType=StackPanel}}" />
        </interactivity:EventTrigger>
    </interactivity:Interaction.Triggers>
</TextBox>

I prefer the first method as I want to put this in a style & apply it to TextBox-es across many xaml-s in an application.