DataGridColumnHeader模板中附加属性不更新的问题排查

问题背景 #

我需要给WPF DataGrid的DataGridColumnHeader制作自定义模板，模板需从附加属性读取数据（比如列筛选状态、标题旋转角度这类）。由于DataGrid列不继承DataContext，我使用BindingProxy类实现跨上下文的数据绑定。目前其他绑定均正常，但唯独模板内的附加属性绑定不响应属性变更——同一个属性用其他方式绑定能实时更新，只有模板里的没反应。

复现代码（.NET Framework 4.7.1） #

XAML部分 #

<Window x:Class="DataGridColumnHeaderBindingIssue.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        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"
        xmlns:local="clr-namespace:DataGridColumnHeaderBindingIssue"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:BindingProxy x:Key="ViewModelProxy" Data="{Binding}" />
        
        <Style x:Key="CustomColumnHeaderStyle" TargetType="{x:Type DataGridColumnHeader}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
                        <Border Background="LightGray" Padding="5">
                            <!-- 此绑定不响应附加属性变更 -->
                            <TextBlock Text="{Binding Path=(local:ColumnProperties.FilterStatus), Source={StaticResource ViewModelProxy}}" />
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <DataGrid ItemsSource="{Binding Items}" AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Name" Binding="{Binding Name}" HeaderStyle="{StaticResource CustomColumnHeaderStyle}" />
                <DataGridTextColumn Header="Value" Binding="{Binding Value}" />
            </DataGrid.Columns>
        </DataGrid>
        <!-- 此绑定可正常响应变更 -->
        <TextBlock Text="{Binding Path=(local:ColumnProperties.FilterStatus), Source={StaticResource ViewModelProxy}}" Margin="10,10,0,0" VerticalAlignment="Top" />
        <Button Content="Toggle Filter Status" Click="Button_Click" Margin="10,40,0,0" VerticalAlignment="Top" />
    </Grid>
</Window>

C#部分 #

using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;

namespace DataGridColumnHeaderBindingIssue
{
    public class BindingProxy : Freezable
    {
        protected override Freezable CreateInstanceCore()
        {
            return new BindingProxy();
        }

        public static readonly DependencyProperty DataProperty =
            DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy));

        public object Data
        {
            get { return GetValue(DataProperty); }
            set { SetValue(DataProperty, value); }
        }
    }

    public static class ColumnProperties
    {
        public static readonly DependencyProperty FilterStatusProperty =
            DependencyProperty.RegisterAttached(
                "FilterStatus",
                typeof(string),
                typeof(ColumnProperties),
                new PropertyMetadata("Not Filtered", OnFilterStatusChanged));

        public static string GetFilterStatus(DependencyObject obj)
        {
            return (string)obj.GetValue(FilterStatusProperty);
        }

        public static void SetFilterStatus(DependencyObject obj, string value)
        {
            obj.SetValue(FilterStatusProperty, value);
        }

        private static void OnFilterStatusChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            // 此处可触发，但模板绑定无响应
        }
    }

    public class ViewModel : INotifyPropertyChanged
    {
        public List<Item> Items { get; set; } = new List<Item>
        {
            new Item { Name = "Item 1", Value = "100" },
            new Item { Name = "Item 2", Value = "200" }
        };

        private string _filterStatus = "Not Filtered";
        public string FilterStatus
        {
            get => _filterStatus;
            set
            {
                _filterStatus = value;
                OnPropertyChanged(nameof(FilterStatus));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
        }
    }

    public class Item
    {
        public string Name { get; set; }
        public string Value { get; set; }
    }

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new ViewModel();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var vm = DataContext as ViewModel;
            vm.FilterStatus = vm.FilterStatus == "Not Filtered" ? "Filtered" : "Not Filtered";
            ColumnProperties.SetFilterStatus(vm, vm.FilterStatus);
        }
    }
}

问题原因分析 #

1. BindingProxy的局限性：默认的BindingProxy仅传递Data对象的引用，不会监听Data对象上附加属性的变更。当附加属性值改变时，Proxy本身不会触发任何变更通知，导致模板内的绑定无法感知变化。
2. 模板绑定的上下文特性：DataGridColumnHeader模板初始化时，绑定的是Proxy的Data引用，但附加属性的变更不会触发Data属性的变更通知，因此绑定不会更新。

解决方案 #

方案1：让BindingProxy实现INotifyPropertyChanged并监听附加属性变更 #

修改BindingProxy类，使其能监听Data对象上的附加属性变化，并主动触发PropertyChanged通知：

public class BindingProxy : Freezable, INotifyPropertyChanged
{
    protected override Freezable CreateInstanceCore()
    {
        return new BindingProxy();
    }

    public static readonly DependencyProperty DataProperty =
        DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null, OnDataChanged));

    public object Data
    {
        get { return GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private static void OnDataChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var proxy = d as BindingProxy;
        proxy?.OnPropertyChanged("Data");

        // 移除旧对象的附加属性监听
        if (e.OldValue != null)
        {
            DependencyPropertyDescriptor.FromProperty(ColumnProperties.FilterStatusProperty, typeof(ViewModel))
                .RemoveValueChanged(e.OldValue, proxy.OnFilterStatusChanged);
        }

        // 为新对象添加附加属性监听
        if (e.NewValue != null)
        {
            DependencyPropertyDescriptor.FromProperty(ColumnProperties.FilterStatusProperty, typeof(ViewModel))
                .AddValueChanged(e.NewValue, proxy.OnFilterStatusChanged);
        }
    }

    private void OnFilterStatusChanged(object sender, EventArgs e)
    {
        // 触发Data属性的变更通知，让绑定更新
        OnPropertyChanged("Data");
    }

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

方案2：调整模板中的绑定路径 #

在模板的绑定中，明确绑定到Proxy的Data对象上的附加属性，并指定Mode=OneWay确保单向绑定更新：

<TextBlock Text="{Binding Path=Data.(local:ColumnProperties.FilterStatus), Source={StaticResource ViewModelProxy}, Mode=OneWay}" />

验证效果 #

修改后点击按钮切换筛选状态，模板内的TextBlock会与页面上的TextBlock同步更新，附加属性的变更通知可正常触发到模板绑定。

内容的提问来源于stack exchange，提问作者Adam Mroczek