为什么 Task.WhenAny() 没有按预期工作

Question

我正在尝试为可等待任务实现超时模式，如下所述：https ://stackoverflow.com/a/11191070

但是，我得到了意想不到的结果。我编写了一个小型控制台应用程序来演示（本文底部）。这些是我得到的场景和结果：

Scenario 1
DelayTask  Thread.Sleep(10000);
ActualTask Thread.Sleep(100);
Invoked with await Task.WhenAny(...
Result: THEN clause invoked  CORRECT

Scenario 2
DelayTask  Thread.Sleep(100);
ActualTask Thread.Sleep(10000);
Invoked with await Task.WhenAny(...
Result: THEN clause invoked  INCORRECT

Scenario 3
DelayTask  Thread.Sleep(10000);
ActualTask Thread.Sleep(100);
Invoked WITHOUT await: Task.WhenAny(...
Result: ELSE clause invoked  INCORRECT

Scenario 4
DelayTask  Thread.Sleep(100);
ActualTask Thread.Sleep(10000);
Invoked WITHOUT await: Task.WhenAny(...
Result: ELSE clause invoked  CORRECT

任何人都可以解释为什么这不能按预期运行。蒂亚！

   class Program
    {

        static void Main(string[] args)
        {
            Run();
        }

        private static async void   Run()
        {
            var _cancelWorkTokenSource = new CancellationTokenSource();
            var task = ActualTask(_cancelWorkTokenSource.Token);
            var delayTask = DelayTask(_cancelWorkTokenSource.Token);

            if (await Task.WhenAny(task, delayTask) == task)
            {
                Console.WriteLine("Task Succeeded in time."); 
            }
            else
            {
                Console.WriteLine("Task exceeded time limit");
            };

            Console.ReadLine();
        }

        private static async Task DelayTask(CancellationToken cancelToken)
        {
            Thread.Sleep(10000);
        }

        private static async Task ActualTask(CancellationToken cancelToken)
        {
            Thread.Sleep(100);
        }
    }

Answer 1

因为您的虚拟方法未正确实现。更改Thread.Sleep为await Task.Delay：

private static async Task DelayTask(CancellationToken cancelToken)
{
    await Task.Delay(10000);
}

 private static async Task ActualTask(CancellationToken cancelToken)
 {
    await Task.Delay(100);
 }

您的代码中现在发生了什么：

1. var task = ActualTask(_cancelWorkTokenSource.Token) 阻塞当前线程 10000 毫秒并返回已完成的任务
2. 在那之后 var delayTask = DelayTask(_cancelWorkTokenSource.Token);阻塞当前线程 100 毫秒并返回完成的任务
3. await Task.WhenAny(task, delayTask) 选择他们中的第一个，因为两者都已完成

添加到您的代码中Console.WriteLine(task.IsCompleted);并Console.WriteLine(delayTask.IsCompleted);在相应的变量初始化后进行检查。