Ok... this is leaving me scratching my head.  I have two WPF controls--one&#39;s a user control and the other&#39;s a custom control.  Let&#39;s call them UserFoo and CustomFoo.  In the control template for CustomFoo, I use an instance of UserFoo which is a named part so I can get to it after the template is applied.  That works fine.

Now both UserFoo and CustomFoo have a `Text` property defined on them (independently, i.e. not a shared DP using AddOwner.  Don&#39;t ask...) that are both declared like this...

    public static readonly DependencyProperty TextProperty = DependencyProperty.Register(
        &quot;Text&quot;,
        typeof(string),
        typeof(UserFoo), // The other is CustomFoo
        new FrameworkPropertyMetadata(
            null,
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
            null,
            null,
            true,
            UpdateSourceTrigger.PropertyChanged
        )
    );

Notice specifically that the mode is set to TwoWay and the UpdateSourceTrigger is set to PropertyChanged, again for both.

So in the style template for CustomFoo, I want to bind CustomFoo&#39;s Text property as the source to the internal UserFoo&#39;s Text property. Normally, this is easy.  You just set UserFoo&#39;s text property to &quot;{TemplateBinding Text}&quot; but for some reason it&#39;s only going one way (i.e. UserFoo is properly set from CustomFoo, but not the reverse), even though again, both DPs are set for two-way!  However, when using a relative source binding instead of a template binding, it works great!  Um... wha??

    // This one works
    Text=&quot;{Binding Text, RelativeSource={RelativeSource AncestorType={local:CustomFoo}}, Mode=TwoWay}&quot;

    // As does this too...
    Text=&quot;{Binding Text, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}&quot;

    // But not this one!
    Text=&quot;{TemplateBinding Text}&quot;


So what gives?  What am I missing?

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In WPF, why doesn't TemplateBinding work where Binding does?