What does Binding="{Binding (0)}" mean?

Question

I found this:

<DataTrigger Value="True" Binding="{Binding (0)}">

triggers in RibbonMenuButton template. What does it mean? I've tried to google, but found nothing.

UPD more code:

<ControlTemplate x:Key="RibbonMenuButtonControlTemplate1" TargetType="{x:Type RibbonMenuButton}">
...
    <DataTrigger Binding="{Binding (0)}" Value="True">
        <Setter Property="TextElement.Foreground" TargetName="MainGrid" Value="{DynamicResource {x:Static SystemColors.MenuTextBrushKey}}"/>
        <Setter Property="PathFill" TargetName="TwoLineText" Value="{DynamicResource {x:Static SystemColors.MenuTextBrushKey}}"/>
        <Setter Property="CornerRadius" TargetName="OuterBorder" Value="0"/>
        <Setter Property="Background" TargetName="OuterBorder" Value="Transparent"/>
        <Setter Property="BorderBrush" TargetName="OuterBorder" Value="Transparent"/>
    </DataTrigger>
...

This code created in Blend after "edit template" button pressed on RibbonMenuButton control.

Answer 1

It looks like a bug. As far as I know, in WPF can not be Binding of this type. Below is a list of deficiencies, please pay attention to the first paragraph, quoted from connect.microsoft.com (archive):

I'm trying to style the ribbon control (System.Windows.Controls.Ribbon) and I keep running into problems.

• If I use Blend or another tool to copy the default template of the Ribbon, the RibbonGroup, the RibbonTab, etc. these templates have errors. There are Bindings like {Binding (0)}.

• I cannot change the height of the ribbon control.

• It's not possible to change the template of the RibbonGroup, to change the appearance of the separator and to place the header in the top left corner of the group.

• It's not possible to add a label to the ApplicationMenuButton. Furthermore this button has a fixed width.

• I can't manage to remove the borders at the bottom of the ribbon control.

As I understand it, there is no hotfix.

Answer 2

The only time this can appear is in the tokenized path format. See the code examples under "Indirect Targeting" in the Storyboards Overview. In XAML code, however, it's invalid. I imagine what happened was something like this:

// something goes wrong here, and GetProperties returns an empty array
var propertyChain = GetProperties();

var path = new PropertyPath("(0)", propertyChain);

trigger.Binding = new Binding { Path = path };

With an empty PathParameters, the XAML generator might then dump the raw tokenized path.