RelativeSource绑定行为不一致原因探究：自定义控件打印异常

Why FindAncestor Fails During Printing (But TemplatedParent Works) #

Great question—this is a classic gotcha with WPF's visual tree behavior during printing vs normal display. Let's break down the root cause:

1. The Visual Tree Difference: Display vs Printing #

When your CustomTextControl is rendered in a Window, the elements in its ControlTemplate are part of the live visual tree directly attached to the control instance. RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:CustomTextControl}} works here because the template's elements are nested directly under the CustomTextControl in the tree, so the ancestor lookup traversal finds it easily.

But when you call System.Windows.Controls.PrintVisual, WPF doesn't use the live visual tree directly. Instead, it creates a temporary, cloned visual tree optimized for printing. This cloned tree often has subtle structural changes:

• Elements might be flattened, wrapped in print-specific containers, or disconnected from their original parent associations.
• The FindAncestor lookup relies on traversing the parent chain in the visual tree, but in the print clone, this chain doesn't include your original CustomTextControl instance (or the traversal path is broken), so the binding fails to locate the ancestor and throws an exception.

2. Why TemplatedParent Is Reliable in This Scenario #

RelativeSource={RelativeSource TemplatedParent} is designed specifically for template scenarios, and it doesn't depend on visual tree traversal. Instead:

• It directly references the control instance that the template is applied to, using a logical association maintained internally by WPF's template system.
• This association stays intact even when the template elements are cloned or moved into the print-specific visual tree. The template knows exactly which control it belongs to, regardless of the current visual tree structure.

Quick Recap #

• FindAncestor = relies on visual tree hierarchy, which breaks in the modified print tree.
• TemplatedParent = relies on template-to-control logical binding, which remains stable through printing.

内容的提问来源于stack exchange，提问作者grek40