由于空值而无法评估绑定时使用默认值 [英] Use a default value when binding cannot be evaluated because of a null value

问题描述

有一种分配特殊值的方法路径？

is there a way to assign a special value when a binding cannot be evaluated because of a null value in the property path ?

例如，如果我在一个类客户中有一个属性名称像这样：

For instance if I have a property Name in a class Customer and a binding like this :

{Binding CurrentCustomer.Name}

当 CurrentCustomer 为null时，我希望绑定生成字符串---。

When CurrentCustomer is null I'd like the binding to produce the string "---".

TargetNullValue和FallbackValue似乎没有办法。

"TargetNullValue" and "FallbackValue" don't seem to do the trick.

提前感谢您的帮助。

编辑：

其实我要做的是用新的源代码代替真实的，当它不可用。
实际情况如下：

In fact what I'm trying to do is substituting a new source value in place of the true when it is not available. The real scenario is the following :

一个bool值用于确定控件的可见性，但是当这个值不可达时，我想将其替换为false。

a bool value is used to determine the visibility of a control, but when this value is not attainable I'd like to replace it with "false".

这是一个完美模仿我的实际用例的图示：

Here is an illustration that perfectly mimics my real use-case :

MainPage.xaml.cs：

MainPage.xaml.cs :

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;

namespace TestSilverlightBindingDefaultValue
{
    public class BoolToVisibilityConverter : IValueConverter
    {
        #region IValueConverter Members

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return (bool)value ? Visibility.Visible : Visibility.Collapsed;
        }

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

        #endregion
    }

    public class Customer
    {
        public bool HasACar
        {
            get;
            set;
        }
    }

    public partial class MainPage : UserControl
    {
        public static readonly DependencyProperty CurrentCustomerProperty =
            DependencyProperty.Register("CurrentCustomer", typeof(Customer), typeof(MainPage), null);

        public Customer CurrentCustomer
        {
            get { return this.GetValue(CurrentCustomerProperty) as Customer; }
            set { this.SetValue(CurrentCustomerProperty, value); }
        }

        public MainPage()
        {
            InitializeComponent();

            this.CurrentCustomer = null;

            this.DataContext = this;
        }
    }
}

MainPage.xaml： p>

MainPage.xaml :

<UserControl x:Class="TestSilverlightBindingDefaultValue.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:local="clr-namespace:TestSilverlightBindingDefaultValue"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="400">
<UserControl.Resources>
    <local:BoolToVisibilityConverter x:Key="boolToVisibilityConverter" />
</UserControl.Resources>
    <StackPanel x:Name="LayoutRoot" Background="White">
    <TextBlock Text="You have a car"  Visibility="{Binding CurrentCustomer.HasACar,Converter={StaticResource boolToVisibilityConverter}}" />
</StackPanel>

FallbackValue 不是解决方案，因为它只会更改生成的值而不是源值。

FallbackValue is not the solution because it would only change the generated value and not the source value.

Abe Heidebrecht 为 WPF 使用 PriorityBinding ，但在 Silverlight 中不存在。

Abe Heidebrecht has provided the perfect solution for WPF with PriorityBinding but it does not exist in Silverlight.

FINAL EDIT ：
Abe Heidebrecht 的第二个解决方案，即包装在另一个元素中，工作正常。

FINAL EDIT : The second solution of Abe Heidebrecht, ie wrapping in another element, is working perfectly.

推荐答案

您可以使用 PriorityBinding ：

<TextBlock>
    <TextBlock.Text>
        <PriorityBinding>
            <Binding Path="CurrentCustomer.Name" />
            <Binding Source="---" />
        </PriorityBinding>
    </TextBlock.Text>
</TextBlock>

好的，对于Silverlight，将元素包装在一个包装器（如Border） 。然后，您将有一个 IValueConverter 将null转换为 Visibility.Collapsed ，其他任何内容可以可见性.Visible ：

Okay, for Silverlight it is a probably easier to wrap the element in a wrapper (like a Border). You then have an IValueConverter to convert null to Visibility.Collapsed, and anything else to Visibility.Visible:

public class NullToVisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value != null ? Visibility.Visible : Visibility.Collapsed;
    }

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

使用它像这样：

<Border Visibility="{Binding CurrentCustomer, Converter={StaticResource NullToVisibilityConverter}}">
    <TextBlock Text="You have a car"  Visibility="{Binding CurrentCustomer.HasACar,Converter={StaticResource boolToVisibilityConverter}}" />
</Border>

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