将 CancellationToken 与异步方法的任务相关联

Question

问题：有没有办法将CancellationToken 与从async 方法返回的Task 关联起来？

通常，如果OperationCancelledException 与CancellationToken 匹配Task 的CancellationToken 被抛出，Task 将最终处于Canceled 状态。如果它们不匹配，则任务进入Faulted 状态：

void WrongCancellationTokenCausesFault()
{
    var cts1 = new CancellationTokenSource();
    var cts2 = new CancellationTokenSource();
    cts2.Cancel();

    // This task will end up in the Faulted state due to the task's CancellationToken
    // not matching the thrown OperationCanceledException's token.
    var task = Task.Run(() => cts2.Token.ThrowIfCancellationRequested(), cts1.Token);  
}

对于async/await，我还没有找到一种方法来设置方法的Task 的CancellationToken（从而实现相同的功能）。从我的测试来看，anyOperationCancelledException 似乎会导致async 方法进入Canceled 状态：

async Task AsyncMethodWithCancellation(CancellationToken ct)
{
    // If ct is cancelled, this will cause the returned Task to be in the Cancelled state
    ct.ThrowIfCancellationRequested(); 
    await Task.Delay(1);

    // This will cause the returned Task to be in the Cancelled state
    var newCts = new CancellationTokenSource();
    newCts.Cancel();
    newCts.Token.ThrowIfCancellationRequested();
}

如果我从async 方法调用的方法被取消（而且我不希望取消——即不是这个Task 的CancellationToken )，我希望任务进入Faulted 状态——而不是Canceled 状态。

Answer 1

我认为该设计适用于常见情况：如果取消任何子操作，则取消传播到父操作（最常见的情况是父子操作共享取消令牌）。

如果你想要不同的语义，你可以在你的 async 方法中 catch OperationCanceledException 并抛出一个符合你需要的语义的异常。如果你想重复使用这些语义，Task 的扩展方法应该适合。

Answer 2

这是一个Run 方法，当提供CancellationToken 参数时，它试图模仿Task.Run 方法的行为。 Run 返回的任务只有在异步方法返回 Canceled 任务并且关联的 CancellationToken 与提供的参数匹配时才能变为 Canceled。

/// <summary>
/// Invokes an async method (a method implemented with the async keyword), and
/// returns a proxy of the produced async task. In case the async task completes
/// in a Canceled state but the causative CancellationToken is not equal with the
/// cancellationToken argument, the proxy transitions to a Faulted state.
/// In all other cases, the proxy propagates the status of the async task as is.
/// </summary>
public static Task<TResult> Run<TResult>(Func<Task<TResult>> asyncMethod,
    CancellationToken cancellationToken)
{
    return asyncMethod().ContinueWith(t =>
    {
        if (t.IsCanceled)
        {
            try { t.GetAwaiter().GetResult(); }
            catch (Exception ex)
            {
                // In case the async method is canceled with an unknown token, throw
                // the exception. The continuation will complete in a Faulted state.
                if (ex is OperationCanceledException oce &&
                    oce.CancellationToken != cancellationToken) throw;
            }
        }

        return t; // In any other case, propagate the task as is.
    }, default, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default)
    .Unwrap();
}

使用示例：

Task<int> task = Run(async () =>
{
    await Task.Delay(1000, new CancellationToken(true));
    return 13;
}, new CancellationToken(false));

try { task.Wait(); } catch { }

Console.WriteLine(task.Status);

输出：

Faulted