Set attached property value on UserControl using element syntax

Question

Suppose I have an attached property "Attached.Template" of type DataTemplate in namespace ns that I wish to set on my UserControl through XAML. Is there a syntax that lets me do this? Here are some things that do not work:

<!-- fails; UserControl may have only one child -->
<UserControl>
   <ns:Attached.Template>
      <DataTemplate />
   </ns:Attached.Template>

   <Grid />
</UserControl>

<!-- fails; the '(' character cannot be included in a name -->
<UserControl>
   <UserControl.(ns:Attached.Template)>
      <DataTemplate />
   </UserControl.(ns:Attached.Template)>

   <Grid />
</UserControl>

<!-- fails; "UserControl.ns" is an undeclared prefix -->
<UserControl>
   <UserControl.ns:Attached.Template>
      <DataTemplate />
   </UserControl.ns:Attached.Template>

   <Grid />
</UserControl>

The property definition is very standard; just following R#'s built-in template:

public static class Attached
{
    public static readonly DependencyProperty TemplateProperty = DependencyProperty.RegisterAttached(
        "Template", typeof(DataTemplate), typeof(Attached),
        new PropertyMetadata(default(DataTemplate)));

    public static void SetTemplate(DependencyObject element, DataTemplate value) =>
        element.SetValue(TemplateProperty, value);

    public static DataTemplate GetTemplate(DependencyObject element) =>
        (DataTemplate) element.GetValue(TemplateProperty);
}

Answer 1

It seems you have to explicitly set the UserControl's Content like this:

<UserControl>
    <ns:Attached.Template>
        <DataTemplate/>
    </ns:Attached.Template>

    <UserControl.Content>
        <Grid/>
    </UserControl.Content>
</UserControl>

This also works:

<UserControl>
    <Grid/>

    <ns:Attached.Template>
        <DataTemplate/>
    </ns:Attached.Template>
</UserControl>

IMO a weird bug or imperfection in the XAML Parser.