In a WPF 4 UserControl I could define the bindings of a sub control in XAML something like this ...

```
...
    <ComboBox 
        ItemsSource="{Binding CBItems, 
                      RelativeSource={RelativeSource AncestorType=UserControl}}"
        ...              
    />
...

```

where CBItems is a property defined in the code behind of the UserControl.

However this format isn't available in Windows 8 Metro - RelativeSource does not have a member AncestorType.

How can I do this type of binding in Windows 8?