This is in the context of
the MVVM pattern for WPF. You can bind to the property of the upper-most datacontext from within a View using RelativeSource and finding the AncestorType=Window's DataContext.
XAML:
<TextBlock Text="{Binding
RelativeSource={RelativeSource FindAncestor,
AncestorType={x:Type Window}}, Path=DataContext.Message}" />