c# - 绑定(bind)到 ViewModel 的 ObservableCollection 类型的附加属性始终返回 null

Question

我正在尝试绑定(bind) ObservableCollection在我的 Window给我的ViewModel使用 Attached Property .绑定(bind)似乎从 PropertyChangedCallBack 开始工作。被调用，但如果我调用 GetMyProperty方法，它总是返回 null .
MainWindow.xaml

<Window x:Class="AttachedOberservableCollectionTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:AttachedOberservableCollectionTest"
        Loaded="Window_Loaded"
        Height="450" Width="800">
    <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
    
    <Grid>
        <local:MainWindow.MyProperty>
            <Binding Path="MyCollection" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged"/>
        </local:MainWindow.MyProperty>
    </Grid>
</Window>

MainWindow.xaml.cs

using System;
using System.Collections.ObjectModel;
using System.Windows;

namespace AttachedOberservableCollectionTest
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        public static ObservableCollection<int> GetMyProperty(DependencyObject obj)
        {
            return (ObservableCollection<int>)obj.GetValue(MyPropertyProperty);
        }

        public static void SetMyProperty(DependencyObject obj, ObservableCollection<int> value)
        {
            obj.SetValue(MyPropertyProperty, value);
        }

        public static readonly DependencyProperty MyPropertyProperty =
            DependencyProperty.RegisterAttached("MyProperty", typeof(ObservableCollection<int>), typeof(MainWindow), new PropertyMetadata(null, MyPopertyChanged));

        private static void MyPopertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var newCollection = (ObservableCollection<int>)e.NewValue;
            Console.WriteLine($"New collection has {newCollection.Count} values");
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            var collection = GetMyProperty(this);

            if (collection == null)
                Console.WriteLine("Attached property is null");
            else
                Console.WriteLine($"Attached poperty has {collection.Count} values");
        }
    }
}

ViewModel.cs

using System.Collections.ObjectModel;

namespace AttachedOberservableCollectionTest
{
    public class ViewModel
    {
        public ObservableCollection<int> MyCollection { get; set; } = new ObservableCollection<int>();

        public ViewModel()
        {
            MyCollection.Add(1);
            MyCollection.Add(2);
        }
    }
}

我得到的输出:

New collection has 2 values
Attached property is null

Visual Studio 中的 xaml 编辑器还提供工具提示消息 The value "System.Windows.Data.Binding" is not of type "System.Int32" and cannot be used in this generic collection. Parameter name: value .

Answer 1

完全不清楚为什么要使用附加属性，而不是 MainWindow 类中的常规依赖属性。
但是，您的 XAML 在顶级 Grid 上设置附加属性，而不是 Window 对象:

<Grid>
    <local:MainWindow.MyProperty>
        <Binding Path="MyCollection" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged"/>
    </local:MainWindow.MyProperty>
</Grid>

实际上应该看起来像

<Grid local:MainWindow.MyProperty="{Binding MyCollection, Mode=TwoWay}">
</Grid>

将分配移动到 Window 实例，例如

<Window ...
    local:MainWindow.MyProperty="{Binding MyCollection, Mode=TwoWay}">

或从网格中检索属性值。

附加属性实际上应该是常规依赖属性。在 MainWindow 的 XAML 中绑定(bind)到它，如下所示:

<Window ...>
    <Window.Style>
        <Style TargetType="local:MainWindow">
            <Setter Property="MyProperty"
                    Value="{Binding MyCollection, Mode=TwoWay}"/>
        </Style>
    </Window.Style>
    ...
</Window>