异步总是 WaitingForActivation [英] Async always WaitingForActivation

问题描述

我想弄清楚 async &await 关键字都是关于，但是输出不是我所期望的.

I am trying to figure out what the async & await keywords are all about, however the output isn't what I'm expecting.

控制台应用如下:

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.";
            });
    }
}

输出为:

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..

我期待在方法启动后看到状态从 WaitingForActivation 改变.

I was expecting to see the status changing from WaitingForActivation once the method is started.

它如何保持这种状态并保持活跃?

How can it stay in this state and be active?

推荐答案

对于我的回答，值得记住的是 TPL (Task-Parallel-Library)、Task 类和 TaskStatus 枚举被引入在 async-await 关键字和 async-await 关键字之前不是 TPL 的原始动机.

For my answer, it is worth remembering that the TPL (Task-Parallel-Library), Task class and TaskStatus enumeration were introduced prior to the async-await keywords and the async-await keywords were not the original motivation of the TPL.

在标记为async的方法的上下文中，产生的Task不是表示方法执行的Task，而是Taskcode>Task 用于继续方法.

In the context of methods marked as async, the resulting Task is not a Task representing the execution of the method, but a Task for the continuation of the method.

这只能使用几种可能的状态:

This is only able to make use of a few possible states:

• 已取消
• 出错
• RanToCompletion
• 等待激活

我知道 Running 似乎是比 WaitingForActivation 更好的默认设置，但是这可能会产生误导，因为在大多数情况下，异步方法是 executed 实际上并没有运行(即它可能是 await-ing 其他东西).另一种选择可能是为 TaskStatus 添加一个新值，但这可能是对现有应用程序和库的重大更改.

I understand that Runningcould appear to have been a better default than WaitingForActivation, however this could be misleading, as the majority of the time, an async method being executed is not actually running (i.e. it may be await-ing something else). The other option may have been to add a new value to TaskStatus, however this could have been a breaking change for existing applications and libraries.

所有这些都与使用 Task.Run 时非常不同，它是原始 TPL 的一部分，它能够使用 TaskStatus 的所有可能值 枚举.

All of this is very different to when making use of Task.Run which is a part of the original TPL, this is able to make use of all the possible values of the TaskStatus enumeration.

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 IProgress(T) 接口的更多信息.

If you wish to keep track of the status of an async method, take a look at the IProgress(T) interface, this will allow you to report the ongoing progress. This blog post, Async in 4.5: Enabling Progress and Cancellation in Async APIs will provide further information on the use of the IProgress(T) interface.

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