单击“取消”按钮时任务未取消[重复]

Question

在我的代码中，我在单击 LoadTemplated 按钮时从 excel 下载数据。 我想在用户单击“取消”按钮后立即取消下载。 不知何故，任务在继续时没有被取消。 需要关于我在这里做错了什么的建议。

public CheckListDetailViewModel(IAuditInspectionDataService auditInspectionDataService)
        {
            _auditInspectionDataService = auditInspectionDataService;

            LoadChecklistTemplateCommand = new DelegateCommand(OnLoadTemplate, CanLoadTemplate).ObservesProperty(() => ChecklistItems.Count);
        }

private CancellationTokenSource cts;        
        private async void OnLoadTemplate()
    {
        try
        {
            if (cts != null)
            {
                cts.Cancel();
            }
            else
            {
                cts = new CancellationTokenSource();
                IsBusy = true;
                LoadTemplateButtonValue = "Cancel";

                var items = await Task.Run(() =>
                {
                    if (cts.Token.IsCancellationRequested)
                        cts.Token.ThrowIfCancellationRequested();

                    var checklistItems = _auditInspectionDataService.GetCheckList(InspectionType.InspectionTypeId);

                    return checklistItems;

                }, cts.Token);

                LoadTemplateButtonValue = "Load Template";
                ChecklistItems = new ObservableCollection<ChecklistItem>(items);

            }


        }
        catch (Exception ex)
        {
            IsBusy = false;
            LoadTemplateButtonValue = "Load Template";
            ChecklistItems = null;
            cts = null;
            Debug.WriteLine(ex);
        }
        finally
        {
            IsBusy = false;
            LoadTemplateButtonValue = "Load Template";
            //cts.Dispose();
        }

    }

Answer 1

当 Task 被取消时，会引发 OperationCanceledException 异常。捕获此异常的 catch 块应该是您在处置已取消的实例之后立即创建 CancellationTokenSource 的新实例的唯一位置。 CancellationTokenSource 只能使用一次（一旦通过调用Cancel() 将CancellationTokenSource.IsCancellationRequested 设置为true）。

只有在调用CancellationToken.ThrowIfCancellationRequested() 并请求取消时，才能取消Task。这意味着CancellationToken 的观察者负责在每次运行操作允许中止时调用CancellationToken.ThrowIfCancellationRequested()，但应尽可能频繁地调用，以便在请求取消时立即取消，并且始终在分配资源之前取消。

因此，只有将当前CancellationToken 的引用传递给在可取消Task 执行期间调用的每个方法才有意义。还要始终确保所有async 方法都返回Task 或Task<T>（除了需要基于event 委托签名返回void 的事件处理程序）。以下示例将按预期工作。

MainWindow.xaml

<Window>
  <Window.DataContext>
    <CheckListDetailViewModel />
  </Window.DataContext>

  <Button>
    <Button.Style>
      <Style>
        <Setter Property="Command" Value="{Binding LoadChecklistTemplateCommand}" />
        <Setter Property="Content" Value="Load Template" />

        <!-- Set the Button's Command to CancelCommand once the download has started -->
        <Style.Triggers>
          <DataTrigger Binding="{Binding IsBusy}" Value="True">
            <Setter Property="Content" Value="Cancel" />
            <Setter Property="Command" Value="{Binding CancelCommand}" />
          </DataTrigger>
        </Style.Triggers>
      </Style>
    </Button.Style>  
  </Button>
</Window>

CheckListDetailViewModel.cs

public CancellationTokenSource CancellationTokenSource { get; set; }  

private bool isBusy;
public bool IsBusy
{ 
  get => this.isBusy;
  set
  {
    this.isBusy = value; 
    OnPropertyChanged();
  }  
}

public ICommand LoadChecklistTemplateCommand { get; set; }
public ICommand CancelDownloadCommand => new DelegateCommand(CancelDownload, () => true);

public CheckListDetailViewModel(IAuditInspectionDataService auditInspectionDataService)
{
  this.CancellationTokenSource = new CancellationTokenSource();

  _auditInspectionDataService = auditInspectionDataService;
  LoadChecklistTemplateCommand = new DelegateCommand(OnLoadTemplate, CanLoadTemplate).ObservesProperty(() => ChecklistItems.Count);
}

// Cancel Task instance
public void CancelDownload()
{
  this.CancellationTokenSource.Cancel();
}

private async Task OnLoadTemplateAsync()
{
  if (this.IsBusy)
  {
    return;
  }

  CancellationToken cancellationToken = this.CancellationTokenSource.Token;
  this.IsBusy = true;

  try
  {
    var items = await Task.Run(() =>
    {
      cancellationToken.ThrowIfCancellationRequested();
      return this._auditInspectionDataService.GetCheckList(cancellationToken, InspectionType.InspectionTypeId);
    }, cancellationToken);

    this.ChecklistItems = new ObservableCollection<ChecklistItem>(items);
  }
  catch (OperationCanceledException) 
  {
    // CancellationTokenSource can only be used once. Therefore dispose and create new instance
    this.CancellationTokenSource.Dispose();
    this.CancellationTokenSource = new CancellationTokenSource();
    this.ChecklistItems = new ObservableCollection<ChecklistItem>();
  }

  this.IsBusy = false;
}

AuditInspectionDataService.cs

// Member of IAuditInspectionDataService 
public void GetCheckList(CancellationToken cancellationToken)
{ 
  using (OleDbConnection connection = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=\"myfile.xls\";Extended Properties=\"Excel 12.0;HDR=YES;IMEX=1\""))
  {
    OleDbCommand command = new OleDbCommand("SELECT * FROM [Sheet1$]", connection);

    connection.Open();
    OleDbDataReader reader = command.ExecuteReader();

    int partitionSize = 50;
    int linesRead = 0;
    while (reader.Read())
    {
      if (++linesRead == partitionSize)
      {
        cancellationToken.ThrowIfCancellationRequested();
        llnesRead = 0;
      }
      Console.WriteLine(reader[0].ToString());
    }
    reader.Close();
  }  
}