无法在多值转换器中访问数据上下文

问题描述

我有一个需要设置特定DataContext的用户控件。用户控件使用多值转换器。但是，多值转换器中的绑定无法使用数据上下文。对于常规值转换器，它工作正常。有人知道发生了什么吗？要澄清的是，下面的代码与我的真实代码不同，它只是一个重现我的问题的示例。例如，我的多值转换器采用了多个参数。

I have a usercontrol which I need to set a specific DataContext on. The usercontrol uses multivalueconverters. However, the binding in multivalueconverter fails to use the datacontext. It works fine for regular value converters. Anyone know what is going on? To clarify, below code is not similar to my real code, it's just a sample reproducing my issue. FOr example, my multivalueconverters take more than one argument.

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>

后面的代码：

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();
        }
    }
}

推荐答案

您不会喜欢它;-）

只需将行 DataContext = this 之前 InitializeComponent（）

似乎第三个文本框的MultiConverter在设置了全局DataContext，实际上产生了空引用错误...

It seems the MultiConverter for the third textbox is triggered before the global DataContext is set, thus in effect producing a null reference error...

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