WPF 文本块 TargetNullValue 不起作用？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/22315699/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
WPF Textblock TargetNullValue not working?
提问by SuicideSheep
I have a wpf textblock as below:
我有一个 wpf 文本块，如下所示：
<TextBlock Text="{Binding [someViewModel].SomeVar.SomeSubVar.Name,
TargetNullValue='-'}"/>
At my viewmodel side, I'll have my own logic that in the end, SomeVar.SomeSubVarwill be null.
在我的视图模型方面，我将拥有自己的逻辑，最终SomeVar.SomeSubVar将为空。
If I want to show a default value for this TextBlockI know I can declare and initiate SomeVar.SomeSubVarand assign default value into SomeVar.SomeSubVar.Namebut I would like to use TargetNullValueinstead. May I know which part is wrong?
如果我想为此显示一个默认值，TextBlock我知道我可以声明和启动SomeVar.SomeSubVar并将默认值分配给，SomeVar.SomeSubVar.Name但我想改为使用TargetNullValue。我可以知道哪个部分有问题吗？
回答by Shawn Rakowski
You might look at using FallbackValue http://msdn.microsoft.com/en-us/library/system.windows.data.bindingbase.fallbackvalue(v=vs.110).aspxin conjunction with TargetNullValue.
您可能会考虑将 FallbackValue http://msdn.microsoft.com/en-us/library/system.windows.data.bindingbase.fallbackvalue(v=vs.110).aspx与 TargetNullValue 结合使用。
In order for TargetNullValue to work you must be able to evaluate the full path of [someViewModel].SomeVar.SomeSubVar.Name to null. If 'someViewModel', 'SomeVar', or 'SomeSubVar' are null then 'Name' can never be evaluated, and TargetNullValue won't apply.
为了使 TargetNullValue 工作，您必须能够将 [someViewModel].SomeVar.SomeSubVar.Name 的完整路径评估为 null。如果“someViewModel”、“SomeVar”或“SomeSubVar”为空，则永远无法评估“Name”，并且 TargetNullValue 将不适用。
回答by Tony Vitabile
The Binding's TargetNullValueproperty is displayed in the TextBlockif the [somveViewModel].SomeVar.SomeSubVar.Nameproperty is null. It doesn't cane the value of the [somveViewModel].SomeVar.SomeSubVar.Nameproperty. It effectively sets up an equivalency between null and the TargetNullValueproperty's value.
该Binding's TargetNullValue属性显示在TextBlock如果[somveViewModel].SomeVar.SomeSubVar.Name属性为null。它不会影响[somveViewModel].SomeVar.SomeSubVar.Name财产的价值。它有效地建立了 null 和TargetNullValue属性值之间的等价关系。
See this questionfor a good explanation.
请参阅此问题以获得很好的解释。
So my advice is if you want to have a non-null default value, you need to set it in the view model.
所以我的建议是如果你想要一个非空的默认值，你需要在视图模型中设置它。
