无法访问多值转换器中的数据上下文

Question

我有一个用户控件，我需要在其上设置特定的 DataContext。用户控件使用多值转换器。但是，multivalueconverter 中的绑定无法使用数据上下文。它对于常规值转换器来说效果很好。有人知道发生了什么事吗？澄清一下，下面的代码与我的真实代码并不相似，它只是重现我的问题的示例。例如，我的多值转换器接受多个参数。

XAML：

<Window x:Class="MultConvTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:MultConvTest="clr-namespace:MultConvTest"
    Title="Window1" Height="300" Width="300">

    <Window.Resources>
        <MultConvTest:MyValueConverter x:Key="MyValueConverter" />
        <MultConvTest:MyMultiConverter x:Key="MyMultiConverter" />
    </Window.Resources>

    <StackPanel>

        <!--This works, output is Formatted: 7-->
        <TextBlock 
            DataContext="{Binding Path=Data}"
            Text="{Binding Path=Length, Converter={StaticResource MyValueConverter}}" />

        <!--This works, output is Formatted: 7-->
        <TextBlock>
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MyMultiConverter}">
                    <Binding Path="Data.Length" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>

        <!--This fails, the converter never access the DataContext of the textbox-->
        <TextBlock 
            DataContext="{Binding Path=Data}">
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MyMultiConverter}">
                    <Binding Path="Length" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>

    </StackPanel>

</Window>

隐藏代码：

using System;
using System.Collections.Generic;
using System.Windows;
using System.Windows.Data;

namespace MultConvTest
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();

            DataContext = this;
        }

        public string Data
        {
            get { return "My Text"; }
        }
    }

    public class MyValueConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            int number = (int)value;
            return string.Format("Formatted: {0}", number);
        }

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

    public class MyMultiConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            // This will fail when DataContext is set. values[0] is of type Ms.Internal.NamedObject 
            // with value DependencyProperty.UnsetValue.
            int number = (int)values[0];
            return string.Format("Formatted: {0}", number);
        }

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

Answer 1

你不会喜欢这个;-)

只需将行 DataContext=this 之前 InitializeComponent()

看起来是第三个文本框的 MultiConverter在设置全局 DataContext 之前触发，从而实际上产生空引用错误...