异步始终等待激活

我想弄清楚是什么async & await关键字是全部，但输出并不是我所期望的。

控制台应用程序如下：

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Foo called");
        var result = Foo(5);

        while (result.Status != TaskStatus.RanToCompletion)
        {
            Console.WriteLine("Thread ID: {0}, Status: {1}", Thread.CurrentThread.ManagedThreadId, result.Status);
            Task.Delay(100).Wait();
        }

        Console.WriteLine("Result: {0}", result.Result);
        Console.WriteLine("Finished.");
        Console.ReadKey(true);
    }

    private static async Task<string> Foo(int seconds)
    {
        return await Task.Run(() =>
            {
                for (int i = 0; i < seconds; i++)
                {
                    Console.WriteLine("Thread ID: {0}, second {1}.", Thread.CurrentThread.ManagedThreadId, i);
                    Task.Delay(TimeSpan.FromSeconds(1)).Wait();
                }

                return "Foo Completed.";
            });
    }
}

输出是：

Foo called
Thread ID: 10, Status: WaitingForActivation
Thread ID: 6, second 0.
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 6, second 1.
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 6, second 2.
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 6, second 3.
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 6, second 4.
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Thread ID: 10, Status: WaitingForActivation
Result: Foo Completed.
Finished..

我期待看到状态从WaitingForActivation一旦该方法启动。

如何才能保持这种状态并保持活跃呢？

对于我的回答，值得记住的是 TPL (任务并行库), Task类和TaskStatus枚举是在 async-await 关键字之前引入的，并且 async-await 关键字并不是 TPL 的原始动机。

在标记为的方法的上下文中async， 所结果的Task不是一个Task代表方法的执行，但是Task为了继续该方法。

这只能利用几种可能的状态：

• Canceled
• Faulted
• 完成任务
• 等待激活

我明白那个Running似乎是一个更好的默认值WaitingForActivation，但是这可能会产生误导，因为大多数情况下，异步方法是executed实际上并没有运行（即它可能是await- 其他的东西）。另一种选择可能是添加一个新值TaskStatus，但是这对于现有应用程序和库来说可能是一个重大变化。

所有这一切都与使用时非常不同Task.Run这是原始 TPL 的一部分，这能够利用所有可能的值TaskStatus枚举。

如果您希望跟踪异步方法的状态，请查看IProgress(T)界面，这将允许您报告正在进行的进度。这篇博文，4.5 中的异步：在异步 API 中启用进度和取消将提供有关使用的更多信息IProgress(T)界面。