WPF ObservableCollection 异步调用问题

问题介绍

当ObservableCollection列表被UI线程占用时，如果在异步线程中调用ObservableCollection，会弹出以下异常：

问题分析

我们使用一个viewModel，在ViewModel中添加ObservableCollection类型的ItemsSource列表。

在列表使用ListBox绑定ItemsSource列表。再由界面触发对ItemsSource的修改。

    public class ViewModel : INotifyPropertyChanged
    {
        private ObservableCollection<string> _itemsSource = new ObservableCollection<string>();

        public ObservableCollection<string> ItemsSource
        {
            get => _itemsSource;
            set
            {
                _itemsSource = value;
                OnPropertyChanged();
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

1. 直接在异步线程下修改ObservableCollection--报错

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
    {
        var viewModel = this.DataContext as ViewModel;
        Task.Run(() =>
        {
           //此段调用异常
            viewModel.ItemsSource.Add("test1");
        });
    }

2. 在异步线程下，赋值ObservableCollection--正常

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
    {
        var viewModel = this.DataContext as ViewModel;
        Task.Run(() =>
        {
            //此段不会报错
            var list = viewModel.ItemsSource.ToList();
            list.Add("test0");
            viewModel.ItemsSource = new ObservableCollection<string>(list);
        });
    }

3. 在异步线程下，赋值ObservableCollection后，再修改ObservableCollection--正常

    private void Button1_OnClick(object sender, RoutedEventArgs e)
    {
        var viewModel = this.DataContext as ViewModel;
        Task.Run(() =>
        {
            //此段不会报错
            viewModel.ItemsSource = new ObservableCollection<string>(new List<string>() { "test3", "test2" });
            //此段不会报错
            viewModel.ItemsSource.Add("test4");
        });
    }

在异步线程下设置的ItemsSource，可以被当前异步线程调用。

4. 异步线程下赋值ObservableCollection，然后在UI线程修改ObservableCollection--正常

        private void Button1_OnClick(object sender, RoutedEventArgs e)
        {
            var viewModel = this.DataContext as ViewModel;
            Task.Run(() =>
            {
                //此段不会报错
                viewModel.ItemsSource = new ObservableCollection<string>(new List<string>() { "test0" });
            });
        }


        private void Button2_OnClick(object sender, RoutedEventArgs e)
        {
            var viewModel = this.DataContext as ViewModel;
            //此段不会报错
            viewModel.ItemsSource.Add("test2");
        }

在异步线程下设置的ItemsSource，可以被UI线程调用。此处可以理解为，赋值时，框架默默转到UI线程处理了？

但是上面3流程，为何正常，so weird~

5. 异步线程下，回到UI线程中，修改ObservableCollection--正常

    private void Button1_OnClick(object sender, RoutedEventArgs e)
    {
        var viewModel = this.DataContext as ViewModel;
        Task.Run(() =>
        {
            Application.Current.Dispatcher.Invoke(() =>
            {
                //此段不会报错
                viewModel.ItemsSource.Add("test");
            });
        });
    }