您不能使用 FindAncestor 将数据绑定到后代的数据...线索就在其名称中.如果子 UserControl 在与父项相同的 XAML 中定义，则您可以为其提供名称，然后使用 Binding.ElementName 属性 将数据绑定到其属性:

You cannot use FindAncestor to data bind to a descendant's data... the clue is in its name. If the child UserControl is defined in the same XAML as the parent, then you can provide it with a name and then use the Binding.ElementName Property to data bind to its properties:

<TextBlock Text="{Binding ChildPropertyName, ElementName=NameOfChildUserControl, 
    UpdateSourceTrigger=PropertyChanged, NotifyOnTargetUpdated=True, Mode=TwoWay}" 
    TargetUpdated="OnTextUpdated" />