wpf 当内容为 null 或为空时，在 ContentControl 中显示默认 DataTemplate？
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/1217254/
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
Display a default DataTemplate in a ContentControl when its content is null or empty?
提问by Bryan Anderson
I would think this is possible, but the obvious way isn't working.
我认为这是可能的，但明显的方法不起作用。
Currently, I'm doing this:
目前，我正在这样做：
<ContentControl
Content="{Binding HurfView.EditedPart}">
<ContentControl.Resources>
<Style
TargetType="ContentControl"
x:Key="emptytemplate">
<Style.Triggers>
<DataTrigger
Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=Content}"
Value="{x:Null}">
<Setter
Property="ContentControl.Template">
<Setter.Value>
<ControlTemplate>
<Grid
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch">
<TextBlock>EMPTY!</TextBlock>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentControl.Resources>
</ContentControl>
I'm not getting any binding errors and this compiles. However, it doesn't produce the expected result. I've also tried the obvious:
我没有收到任何绑定错误并且可以编译。但是，它不会产生预期的结果。我也试过显而易见的：
<DataTemplate DataType="{x:Null}"><TextBlock>Hurf</TextBlock></DataTemplate>
This won't compile. And attempting to set the content twice fails as well:
这不会编译。尝试设置内容两次也失败了：
<ContentControl
Content="{Binding HurfView.EditedPart}">
<TextBlock>DEFAULT DISPLAY</TextBlock>
</ContentControl>
Can I do this without writing a custom template selector?
我可以在不编写自定义模板选择器的情况下执行此操作吗？
回答by Bryan Anderson
Simple, you have to bind the content property in the style. Styles won't overwrite a value on a control if there's a binding present, even if the value evaluates to Null. Try this.
很简单，你必须在样式中绑定 content 属性。如果存在绑定，样式不会覆盖控件上的值，即使该值的计算结果为 Null。尝试这个。
<ContentControl>
<ContentControl.Style>
<Style TargetType="ContentControl">
<Setter Property="Content" Value="{Binding HurfView.EditedPart}" />
<Style.Triggers>
<DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=Content}" Value="{x:Null}">
<Setter Property="ContentControl.Template">
<Setter.Value>
<ControlTemplate>
<Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<TextBlock>EMPTY!</TextBlock>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentControl.Style>
</ContentControl>
回答by Herm
Since I stumbled upon this question and had the same problem today, I wanted to contribute another way how I solved the problem. Since I did not like to add another style trigger I used the property TargetNullValuewhich seems to be a bit more readable than the accepted solution (which works nevertheless):
由于我偶然发现了这个问题并且今天遇到了同样的问题，我想贡献另一种解决问题的方式。由于我不喜欢添加另一个样式触发器，因此我使用了TargetNullValue似乎比接受的解决方案更具可读性的属性（尽管如此）：
<ContentControl>
<ContentControl.Content>
<Binding Path="ContentViewModel">
<Binding.TargetNullValue>
<Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<TextBlock>EMPTY!</TextBlock>
</Grid>
</Binding.TargetNullValue>
</Binding>
</ContentControl.Content>
</ContentControl>
回答by Thomas Levesque
You could return DBNull.Value as the FallbackValue of the Binding for the Content of the ContentControl, and create a DataTemplate for DBNull :
您可以返回 DBNull.Value 作为 ContentControl 内容绑定的 FallbackValue，并为 DBNull 创建一个 DataTemplate ：
<DataTemplate DataType="{x:Type system:DBNull}">
<!-- The default template -->
</DataTemplate>
...
<ContentControl Content="{Binding HurfView.EditedPart, FallbackValue={x:Static system:DBNull.Value}}" />
