TPL任务继续：任务处于故障状态，而不是被取消时取消

Question

取消以下任务时，该任务的状态不是“已取消但有故障：

    private string ReturnString()
    {
        // throw new OperationCanceledException(_cancellationToken);   // This puts task in faulted, not canceled
        Task.Delay(5000, _cancellationToken).Wait(_cancellationToken); // Simulate work (with IO-bound call)
        // throw new OperationCanceledException(_cancellationToken);   // This puts task in faulted, not canceled
        // _cancellationToken.ThrowIfCancellationRequested();          // This puts task in faulted, not canceled  
        // throw new Exception("Throwing this exception works!");      // This works as expected (faulted)
        return "Ready";
    }

    private void SetReturnValueWithTaskContinuation()
    {
        SynchronizationContext synchronizationContext = SynchronizationContext.Current;
        Task<string> task = Task.Run(() => ReturnString());
        task.ContinueWith(
        antecedent =>
        {
            if (antecedent.Status == TaskStatus.Canceled)
            {
                synchronizationContext.Post(result => _txtResultContinueWith.Text = (string)result, "Cancelled");
            }
            else if (antecedent.Status == TaskStatus.Faulted)
            {
                synchronizationContext.Post(result => _txtResultContinueWith.Text = (string)result, "Exception");
            }
            else
            {
                synchronizationContext.Post(result => _txtResultContinueWith.Text = (string)result, antecedent.Result);
            }
        });
    }

我知道，抛出OperationCanceled异常时必须提供取消令牌。我知道，有两种引发OperationCanceled异常的方式，其中ThrowIfCancellationRequested（）是首选方式。而且我知道，延续链的取消令牌应该与要取消的任务的取消令牌不同，否则继续链也会被取消。为了简化起见，我仅使用一个取消令牌来取消任务本身。但是，该任务的状态为“ Faulted”而不是“ Canceled”。那是个错误吗？如果不是，那么这就是TPL的可用性问题。有人可以帮忙吗？

Answer 1

[Task.Run确实希望我们提供取消令牌以正确传播取消状态，请参阅：

Faulted vs Canceled task status after CancellationToken.ThrowIfCancellationRequested

如果我们使用接受Task.Run或Action委托的Func<T>替代，其中T不为Task的任何东西，则这尤其重要。如果没有令牌，则返回的任务状态将是Faulted，而不是Canceled。

但是，如果委托类型是Func<Task> / Func<Task<T>（例如async lambda），则会得到一些特殊待遇。委托返回的任务将被解包，并且其取消状态将正确传播。因此，如果我们按如下所示更改ReturnString，您将获得预期的Canceled状态，并且不必将令牌传递给Task.Run：

private Task<string> ReturnString()
{
    // throw new OperationCanceledException(_cancellationToken);   // This puts task in faulted, not canceled
    Task.Delay(5000, _cancellationToken).Wait(_cancellationToken); // Simulate work (with IO-bound call)
    // throw new OperationCanceledException(_cancellationToken);   // This puts task in faulted, not canceled
    // _cancellationToken.ThrowIfCancellationRequested();          // This puts task in faulted, not canceled  
    // throw new Exception("Throwing this exception works!");      // This works as expected (faulted)
    return Task.FromResult("Ready");
}

// ...

Task<string> task = Task.Run(() => ReturnString()); // Canceled status gets propagated

[更好的是，如果有可能使ReturnString自然地异步，那可能是最好的选择，甚至可能不需要用Task.Run包装它（但如果这样做，取消传播仍然会工作）：

private async Task<string> ReturnString()
{
    // throw new OperationCanceledException(_cancellationToken);   // This puts task in faulted, not canceled
    await Task.Delay(5000, _cancellationToken); // Simulate work (with IO-bound call)
    // throw new OperationCanceledException(_cancellationToken);   // This puts task in faulted, not canceled
    // _cancellationToken.ThrowIfCancellationRequested();          // This puts task in faulted, not canceled  
    // throw new Exception("Throwing this exception works!");      // This works as expected (faulted)
    return "Ready";
}

如果对Task.Run为何如此工作感到好奇，可以深入研究implementation details。