如果属性路径中的父对象为空怎么办? [英] What if parent object in property path is null?

问题描述

如果属性路径中的父对象之一为null，我们如何控制会发生什么?例如:

How do we control what happens if one of the parent objects in the property path is null? For example:

<Button Command="{Binding ActiveDrawing.PrintCommand}" />

如果ActiveDrawing为空怎么办?在这种情况下，我希望禁用此按钮，但是WPF使其保持启用状态.我尝试将FallBackValue设置为null，如下所示:

What if ActiveDrawing is null? I want this button to be disabled in that case, but WPF keeps it enabled. I have tried setting FallBackValue to null, like this:

<Button Command="{Binding ActiveDrawing.PrintCommand, FallbackValue={x:Null}}" />

但是没有什么区别.该按钮保持启用状态.

but it doesn't make a difference. The button keeps enabled.

将TargetNullValue设置为{x:Null}也没有任何区别.

N.B. Setting TargetNullValue to {x:Null} also doesn't make a difference.

推荐答案

我目前已经设计了以下解决方法.

I have devised the following workaround for now.

1. 创建一个名为NullCommand的新类:

Public Class NullCommand
  Implements ICommand

  Public Event CanExecuteChanged As EventHandler Implements ICommand.CanExecuteChanged

  Public Sub Execute(parameter As Object) Implements ICommand.Execute
  End Sub

  Public Function CanExecute(parameter As Object) As Boolean Implements ICommand.CanExecute
    Return False
  End Function
End Class

在资源"部分中创建该类的实例:

Create an instance of the class in the Resources section:

<Window.Resources>
  <vm:NullCommand x:Key="NullCommand" />
</RibbonGroup.Resources>

将此对象用作您的FallbackValue:

Use this object as your FallbackValue:

<Button Command="{Binding ActiveDrawing.PrintCommand, FallbackValue={StaticResource NullCommand}" />

好啦！有用.每当绑定属性路径由于任何原因而失败时，您的按钮都将被禁用.

Hurrah! It works. Whenever the binding property path fails for any reason, your button will be disabled.

TBH，由于一个唯一的原因，我不喜欢这种解决方案. FallbackValue应该已经处理了这种情况.

TBH, I don't like this solution for one sole reason. FallbackValue should have handled this situation.

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