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");
             });
         });
     }