Question

我在将项目添加到通过异步等待在后台线程中创建的ObservableCollection时遇到问题。我将MenuItemViewModels绑定到TreeView ContextMenu中的分层数据模板，该模板在为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都从一个MenuItemBaseViewModel派生而来，该MenuItemBaseViewModel也包含一个ObservableCollection子项，以便使用HierachicalDataTemplate创建一个层次菜单结构：

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，通过NotifyPropertyChanged在MenuItemBaseViewModel中更新MenuItemSource时，通过异步等待加载MenuItems给了我一个例外：

System.InvalidOperationException： 调用线程无法访问该对象，因为不同线程拥有它。

（请参阅代码中的注释）

将ContextMenu绑定到TreeNodesViewModel中的MenuItems-Collection不会给出异常，因为Collection是在UI-Thread上创建的，但是绑定到MenuItemBaseViewModel中的第二层层次SubItems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，并且我不断收到错误消息。我还尝试实现AsyncObservableCollection（https://www.thomaslevesque.com/2009/04/17/wpf-binding-to-an-asynchronous-collection/），但没有成功。有人可以帮我吗？

Answer 1

如评论中的Mike Zboray所示，需要在正确的线程中创建ICollectionView。更改MenuItemBaseViewModel中ICollectionView的吸气剂可解决此问题，因为以前的ICollectionView已在后台线程中创建。返回一个新创建的文件可以为我解决问题：

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时发生异常

1 个答案: