XAML Data Binding to another Class

Question

What is the best way to bind Tag Attribute of ChildButton in TreeView(in the code Tag="{Binding ParentData}" to ParentData property of class TreeParent?

<TreeView>
  <TreeView.Resources>
    <HierarchicalDataTemplate DataType="{x:Type local:TreeParent}" ItemsSource="{Binding Members}">
        <Button x:Name="ParentButton" Text="{Binding ParentName}">
        </Button>
    </HierarchicalDataTemplate>

    <DataTemplate DataType="{x:Type local:TreeChild}" ItemsSource="{Binding Members}">
        <Button x:Name="ChildButton" Text="{Binding ChildName}" Tag="{Binding ParentData}">
        </Button>
    </DataTemplate>
  </TreeView.Resources>
</TreeView>

And the .cs code behind xaml file looks like this:

public class TreeParent
{
   public string ParentName { get; set; }
   public SomeDataClass ParentData { get; set; }
   public ObservableCollection<TreeChild> Members { get; set; }
}

public class TreeChild
{
   public string ChildName { get; set; }
}

Answer 1

Try this:

<Button x:Name="ChildButton" ... 
        Tag="{Binding DataContext.ParentData, 
             RelativeSource={RelativeSource AncestorType=TreeViewItem, AncestorLevel=2}}" />