問題介紹

當ObservableCollection列表被UI線程占用時，如果在異步線程中調用ObservableCollection，會彈出以下異常：

問題分析

我們使用一個viewModel，在ViewModel中添加ObservableCollection類型的ItemsSource列表。

在列表使用ListBox綁定ItemsSource列表。再由界面觸發對ItemsSource的修改。

 1     public class ViewModel : INotifyPropertyChanged
 2     {
 3         private ObservableCollection<string
 
> _itemsSource = new ObservableCollection<string>();
 4 
 5         public ObservableCollection<string> ItemsSource
 6         {
 7             get => _itemsSource;
 8             set
 9             {
10                 _itemsSource = value;
11                 OnPropertyChanged();
 
12             }
13         }
14 
15         public event PropertyChangedEventHandler PropertyChanged;
16 
17         [NotifyPropertyChangedInvocator]
18         protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
19         {
20             PropertyChanged?.Invoke(this
 
, new PropertyChangedEventArgs(propertyName));
21         }
22     }

1. 直接在異步線程下修改ObservableCollection--報錯

1     private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
2     {
3         var viewModel = this.DataContext as ViewModel;
4         Task.Run(() =>
5         {
6            //此段調用異常
7             viewModel.ItemsSource.Add("test1");
8         });
9     }

2. 在異步線程下，賦值ObservableCollection--正常

 1     private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
 2     {
 3         var viewModel = this.DataContext as ViewModel;
 4         Task.Run(() =>
 5         {
 6             //此段不會報錯
 7             var list = viewModel.ItemsSource.ToList();
 8             list.Add("test0");
 9             viewModel.ItemsSource = new ObservableCollection<string>(list);
10         });
11     }

3. 在異步線程下，賦值ObservableCollection後，再修改ObservableCollection--正常

 1     private void Button1_OnClick(object sender, RoutedEventArgs e)
 2     {
 3         var viewModel = this.DataContext as ViewModel;
 4         Task.Run(() =>
 5         {
 6             //此段不會報錯
 7             viewModel.ItemsSource = new ObservableCollection<string>(new List<string>() { "test3", "test2" });
 8             //此段不會報錯
 9             viewModel.ItemsSource.Add("test4");
10         });
11     }

在異步線程下設置的ItemsSource，可以被當前異步線程調用。

4. 異步線程下賦值ObservableCollection，然後在UI線程修改ObservableCollection--正常

 1         private void Button1_OnClick(object sender, RoutedEventArgs e)
 2         {
 3             var viewModel = this.DataContext as ViewModel;
 4             Task.Run(() =>
 5             {
 6                 //此段不會報錯
 7                 viewModel.ItemsSource = new ObservableCollection<string>(new List<string>() { "test0" });
 8             });
 9         }
10 
11 
12         private void Button2_OnClick(object sender, RoutedEventArgs e)
13         {
14             var viewModel = this.DataContext as ViewModel;
15             //此段不會報錯
16             viewModel.ItemsSource.Add("test2");
17         }

在異步線程下設置的ItemsSource，可以被UI線程調用。此處可以理解為，賦值時，框架默默轉到UI線程處理了？

但是上面4流程，為何正常，so weird~

5. 異步線程下，回到UI線程中，修改ObservableCollection--正常

 1     private void Button1_OnClick(object sender, RoutedEventArgs e)
 2     {
 3         var viewModel = this.DataContext as ViewModel;
 4         Task.Run(() =>
 5         {
 6             Application.Current.Dispatcher.Invoke(() =>
 7             {
 8                 //此段不會報錯
 9                 viewModel.ItemsSource.Add("test");
10             });
11         });
12     }

WPF ObservableCollection 異步調用問題