WPF - How to set DataContext on virtual branch of logical tree?

Question

How do I set the DataContext of "myElement" to the same DataContext as the user control in XAML? I have been able to do it in code-behind via myElement.DataContext = this.DataContext in the UserControl constructor, but I'm trying to understand how to do the same thing in XAML.

This is related to creating virtual branches to the logical tree (see this article).

<UserControl ... DataContext="{Binding RelativeSource={RelativeSource self}}">
    ...
    <TextBox Name="myTextBox">    
        <TextBox.Text>        
            <Binding Path="MySource" UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>             
                    <base:StringEqualsRules>
                        <base:StringEqualRule.BoundElement>
                            <base:ValidationRuleElement 
                                x:Name="myElement"
                                DataContext="???"
                                Value="{Binding MyProperty}" />
                        </base:StringEqualRule.BoundElement>
                    </base:StringEqualsRule>
                 </Binding.ValidationRules>
             </Binding>    
         </TextBox.Text>
    </TextBox>
    ...
</UserControl>

Answer 1

Give the top-level UserControl element an x:Name then use an ElementName binding. Like so:

<UserControl ... x:Name="Root" DataContext="...">
    <SomeNestedChild DataContext="{Binding DataContext, ElementName=Root}" />
</UserControl>

Answer 2

Using code-behind turned out to be the easiest and cleanest solution I've found so far:

myElement.DataContext = this.DataContext