WPF：绑定到已异步填充的 ObservableCollection 时出现异常

Question

我在向通过异步等待在后台线程中创建的 ObservableCollection 添加项目时遇到问题。我将 MenuItemViewModels 绑定到 TreeView ContextMenu 中的分层数据模板，这些模板在为 Treeview 编写的 ViewModel 中动态加载（TreeNodesViewModel - > LoadMenuItemsAsync; UI-Thread）：

public class TreeNodesViewModel: BaseViewModel {
    private FullyObservableCollection<MenuItemBaseViewModel> menuitems = new FullyObservableCollection<MenuItemBaseViewModel>();
    private CollectionViewSource viewsourcemenuitems = new CollectionViewSource();

    public FullyObservableCollection<MenuItemBaseViewModel> MenuItems {
        get { return menuitems; }
        set { SetProperty(ref menuitems, value); }
    }

    public TreeNodesViewModel() {
        GenerateCompleteTree();
    }

    public void GenerateCompleteTree(bool setSelectLastSelectedItem = true, string path = null) {
        LoadMenuItemsAsync();
        ...
    }

    public async void LoadMenuItemsAsync() {
        Task load = Task.Run(async () => {
            foreach (Type type in Assembly.GetExecutingAssembly().GetTypes().Where(x => x.IsClass && x.BaseType == typeof(MenuItemBaseViewModel) && x.Namespace.Equals("***.UI.Tree.MenuItems"))) {
                MenuItems.Add((MenuItemBaseViewModel)Activator.CreateInstance(type));
            }
        });

        await load;
    }
}

每个 MenuItemViewModel 派生自 MenuItemBaseViewModel，它还包含一个 ObservableCollection 子项，以便使用 HierachicalDataTemplate 创建一个 Hierarchical Menustructure：

public class MenuItemAddNewSystem : MenuItemBaseViewModel {

    public MenuItemAddNewSystem() : base(PackIconModernKind.Add, Colors.Green) {
        Uid = "...";
        Header = "Add new System";
    }

    #region Overrides
    public override bool IsVisible {
        get {
            return true;
        }
    }


    public override void ExecuteMouseLeftButtonDownCommand(object parameter) {
        throw new NotImplementedException();
    }
    #endregion
}


public abstract class MenuItemBaseViewModel : BaseViewModel {
    private FullyObservableCollection<MenuItemBaseViewModel> subitems = new FullyObservableCollection<MenuItemBaseViewModel>();
    private CollectionViewSource viewsource = new CollectionViewSource();

    public FullyObservableCollection<MenuItemBaseViewModel> SubItems {
        get { return subitems; }
        set { if (SetProperty(ref subitems, value)) OnPropertyChanged("MenuItemSource"); }
    }

    public ICollectionView MenuItemSource {
        get {
            if (SubItems.Count(x => x.IsVisible) == 0) return null;
            viewsource.Source = SubItems.Where(x => x.IsVisible); //<--- Exception here!
            return viewsource.View;
        }
    }

    public MenuItemBaseViewModel(Enum packiconkind, Color? iconcolor = null) {
        SetPackIcon(packiconkind, iconcolor);
        AddEventHandler(SubItems);
        ...
    }

    #region NotifyChanged-Events
    public override void NotifyCollectionChanged(object sender, NotifyCollectionChangedEventArgs e) {
        OnPropertyChanged("MenuItemSource");
    }

    public override void NotifyItemPropertyChanged(object sender, ItemPropertyChangedEventArgs e) {
        if (e.PropertyName.Equals("IsVisible")) OnPropertyChanged("MenuItemSource");
    }
    #endregion

    ...
}

XAML:

<ContextMenu x:Key="MenuItemContextMenu" ItemsSource="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}, Path=TreeViewModel.MenuItemSource}">
    <ContextMenu.ItemTemplate>
        <HierarchicalDataTemplate DataType="{x:Type model:MenuItemBaseViewModel}" ItemsSource="{Binding Path=MenuItemSource, UpdateSourceTrigger=PropertyChanged}">
            <TextBlock Text="{Binding Header}"/>
        </HierarchicalDataTemplate>
    </ContextMenu.ItemTemplate>
    <ContextMenu.ItemContainerStyle>
        <Style TargetType="MenuItem">
            <Setter Property="ToolTip" Value="{Binding ToolTip}"/>
            <Setter Property="IsEnabled" Value="{Binding IsEnabled}"/>
            <Setter Property="Command" Value="{Binding MouseLeftButtonDownCommand}"/>
        </Style>
    </ContextMenu.ItemContainerStyle>
</ContextMenu>

我的问题是，在通过 NotifyPropertyChanged 更新 MenuItemBaseViewModel 中的 MenuItemSource 时，通过异步等待加载 MenuItems 会出现异常：

System.InvalidOperationException: The calling thread cannot access this object because a different thread owns it.

（见代码注释）

将 ContextMenu 绑定到 TreeNodesViewModel 中的 MenuItems-Collection 不会出现异常，因为该 Collection 是在 UI-Thread 上创建的，但是绑定到 MenuItemBaseViewModel 中的第二层子项会引发异常。我尝试了以下选项：

在添加或删除操作期间使用 BindingOperations.EnableCollectionSynchronization 和锁定
使用 Application.Current.Dispatcher
使用viewsource.Dispatcher.BeginInvoke
使用 Dispatcher.CurrentDispatcher.BeginInvoke

没有任何效果，而且我不断收到错误消息。我还尝试实施 AsyncObservableCollection (https://www.thomaslevesque.com/2009/04/17/wpf-binding-to-an-asynchronous-collection/) 但没有成功。谁能帮我解决这个问题？

Answer 1

如评论中Mike Zboray所述，需要在正确的线程中创建 ICollectionView。更改 MenuItemBaseViewModel 中 ICollectionView 的 getter 解决了这个问题，因为之前的 ICollectionView 是在后台线程中创建的。返回新创建的一个解决了我的问题：

public ICollectionView MenuItemSource {
    get { return new CollectionViewSource { Source = SubItems.Where(x => x.IsVisible) }.View; }
}

WPF：绑定到已异步填充的 ObservableCollection 时出现异常

WPF: Exception when Binding to an ObservableCollection that has been filled asynchronously

c#

wpf

observablecollection

async-await