如何取消等待中的任务? [英] How to cancel a Task in await?

问题描述

我正在处理这些 Windows 8 WinRT 任务，并且我正在尝试使用以下方法取消任务，并且它在某种程度上起作用.CancelNotification 方法确实被调用了，这让你认为任务被取消了，但在后台任务一直在运行，然后任务完成后，任务的状态始终是完成的，永远不会被取消.有没有办法在任务取消时完全停止任务?

I'm playing with these Windows 8 WinRT tasks, and I'm trying to cancel a task using the method below, and it works to some point. The CancelNotification method DOES get called, which makes you think the task was cancelled, but in the background the task keeps running, then after it's completed, the status of the Task is always completed and never cancelled. Is there a way to completely halt the task when it's cancelled?

private async void TryTask()
{
    CancellationTokenSource source = new CancellationTokenSource();
    source.Token.Register(CancelNotification);
    source.CancelAfter(TimeSpan.FromSeconds(1));
    var task = Task<int>.Factory.StartNew(() => slowFunc(1, 2), source.Token);

    await task;            

    if (task.IsCompleted)
    {
        MessageDialog md = new MessageDialog(task.Result.ToString());
        await md.ShowAsync();
    }
    else
    {
        MessageDialog md = new MessageDialog("Uncompleted");
        await md.ShowAsync();
    }
}

private int slowFunc(int a, int b)
{
    string someString = string.Empty;
    for (int i = 0; i < 200000; i++)
    {
        someString += "a";
    }

    return a + b;
}

private void CancelNotification()
{
}

推荐答案

阅读 取消(在 .NET 4.0 中引入，从那时起基本没有变化)和 基于任务的异步模式，它提供了有关如何将 CancellationToken 与 async 方法一起使用的指南.

Read up on Cancellation (which was introduced in .NET 4.0 and is largely unchanged since then) and the Task-Based Asynchronous Pattern, which provides guidelines on how to use CancellationToken with async methods.

总而言之，您将 CancellationToken 传递到每个支持取消的方法中，并且该方法必须定期检查它.

To summarize, you pass a CancellationToken into each method that supports cancellation, and that method must check it periodically.

private async Task TryTask()
{
  CancellationTokenSource source = new CancellationTokenSource();
  source.CancelAfter(TimeSpan.FromSeconds(1));
  Task<int> task = Task.Run(() => slowFunc(1, 2, source.Token), source.Token);

  // (A canceled task will raise an exception when awaited).
  await task;
}

private int slowFunc(int a, int b, CancellationToken cancellationToken)
{
  string someString = string.Empty;
  for (int i = 0; i < 200000; i++)
  {
    someString += "a";
    if (i % 1000 == 0)
      cancellationToken.ThrowIfCancellationRequested();
  }

  return a + b;
}

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