当量.在XAML绑定中的Coalesce()？

Question

在SQL中我可以这样做:

Select Coalesce(Property1, Property2, Property3, 'All Null') as Value
From MyTable 

如果Property1,2和3都为null,那么我得到'All Null'

我如何在XAML中执行此操作？我试过以下,但没有运气:

<Window.Resources>
    <local:Item x:Key="MyData" 
                Property1="{x:Null}"
                Property2="{x:Null}"
                Property3="Hello World" />
</Window.Resources>

<TextBlock DataContext="{StaticResource MyData}">
    <TextBlock.Text>
        <PriorityBinding TargetNullValue="All Null">
            <Binding Path="Property1" />
            <Binding Path="Property2" />
            <Binding Path="Property3" />
        </PriorityBinding>
    </TextBlock.Text>
</TextBlock>

结果应该是'Hello World',而是'All Null'

我希望我的问题很明确.

Answer 1

您必须构建一个自定义IMul​​tiValueConverter才能执行此操作并使用MultiBinding.PriorityBinding使用集合中的第一个绑定成功生成值.在您的情况下,Property1绑定立即解析,因此使用它.由于Property1为null,因此使用TargetNullValue.

像这样的转换器:

public class CoalesceConverter : System.Windows.Data.IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, 
            object parameter, System.Globalization.CultureInfo culture)
    {
        if (values == null)
            return null;
        foreach (var item in values)
            if (item != null)
                return item;
        return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, 
            object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

和MultiBinding像这样:

<Window.Resources>
    <local:Item x:Key="MyData" 
                Property1="{x:Null}"
                Property2="{x:Null}"
                Property3="Hello World" />
    <local:CoalesceConverter x:Key="MyConverter" />
</Window.Resources>

<TextBlock DataContext="{StaticResource MyData}">
    <TextBlock.Text>
        <MultiBinding Converter="{StaticResource MyConverter}">
            <Binding Path="Property1" />
            <Binding Path="Property2" />
            <Binding Path="Property3" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>