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问异步程序不异步运行的奇怪行为
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Stack Overflow用户

提问于 2020-04-15 15:46:24

回答 1查看 104关注 0票数 0

我在下面的程序中打印了哪个线程执行每个被调用的方法，结果非常奇怪。我希望所有异步调用都由线程池线程执行，因为@Jon提到异步调用使用threadpool线程，线程池线程是后台线程。

线程id显示，执行Main方法的同一个线程执行除DisplayResult以外的所有调用的方法，后者不是异步方法。

为什么不是异步方法的DisplayResult的每个调用都由线程池线程执行，当调用时实际的异步方法是由非后台线程执行的，与执行主方法的非后台线程相同？在这个程序中应该有多少后台线程和非后台线程？

public static async Task Main()
        {
            Console.WriteLine($"Main method: The thread executing this task is {Thread.CurrentThread.Name}, {Thread.CurrentThread.ManagedThreadId}");

            if (Thread.CurrentThread.IsBackground)
            {
                Console.WriteLine($"And it is a background thread.");

            }
            else
            {
                Console.WriteLine("And it is a non-background thread");
            }

            Console.WriteLine();

            if (Thread.CurrentThread.IsThreadPoolThread)
            {
                Console.WriteLine("It is a ThreadPoolThread");
            }
            else
            {
                Console.WriteLine("It is a non-ThreadPoolThread");
            }

            Task task1 = ProcessReadWriteAsync(@"/tmp/temp1Write.txt");

            Task task2 = ProcessReadWriteAsync(@"/tmp/temp2Write.txt");

            await task1;
            await task2;
        }

        public static async Task ProcessReadWriteAsync(string filePath)
        {
            Console.WriteLine($"ProcessReadWriteAsync: The thread executing this task is {Thread.CurrentThread.Name}, {Thread.CurrentThread.ManagedThreadId}");
            if (Thread.CurrentThread.IsBackground)
            {
                Console.WriteLine($"And it is a background thread.");

            }
            else
            {
                Console.WriteLine("And it is a non-background thread");
            }

            Console.WriteLine();

            if (Thread.CurrentThread.IsThreadPoolThread)
            {
                Console.WriteLine("It is a ThreadPoolThread");
            }
            else
            {
                Console.WriteLine("It is a non-ThreadPoolThread");
            }

            try
            {
            await ReadWriteAsync(filePath);

            } catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            } finally
            {
                Console.WriteLine();
            }
        }

        public static async Task ReadWriteAsync(string path, string text)
        {
            Console.WriteLine($"ReadWriteAsync 2 parameters: The thread executing this task is {Thread.CurrentThread.Name}, {Thread.CurrentThread.ManagedThreadId}");

            if (Thread.CurrentThread.IsBackground)
            {
                Console.WriteLine($"And it is a background thread.");

            }
            else
            {
                Console.WriteLine("And it is a non-background thread");
            }

            Console.WriteLine();

            if (Thread.CurrentThread.IsThreadPoolThread)
            {
                Console.WriteLine("It is a ThreadPoolThread");
            }
            else
            {
                Console.WriteLine("It is a non-ThreadPoolThread");
            }

            FileStream stream = new FileStream(path, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite, 4096, useAsync: true);

                byte[] buffer = new byte[0x1000];

                int noOfCharactersRead = await stream.ReadAsync(buffer, 0, buffer.Length);

                DisplayResult(buffer: buffer);
        }

        public static async Task ReadWriteAsync(string path)
        {
            Console.WriteLine($"ReadWriteAsync 1 parameters: The thread executing this task is {Thread.CurrentThread.Name}, {Thread.CurrentThread.ManagedThreadId}");
            if (Thread.CurrentThread.IsBackground)
            {
                Console.WriteLine($"And it is a background thread.");

            }
            else
            {
                Console.WriteLine("And it is a non-background thread");
            }

            Console.WriteLine();

            if (Thread.CurrentThread.IsThreadPoolThread)
            {
                Console.WriteLine("It is a ThreadPoolThread");
            }
            else
            {
                Console.WriteLine("It is a non-ThreadPoolThread");
            }

            await ReadWriteAsync(path, "");
        }

        private static void DisplayResult(byte[] buffer)
        {
            Console.WriteLine($"DisplayResult: The thread executing this method is {Thread.CurrentThread.Name}, {Thread.CurrentThread.ManagedThreadId}");
            if (Thread.CurrentThread.IsBackground)
            {
                Console.WriteLine($"And it is a background thread.");

            }
            else
            {
                Console.WriteLine("And it is a non-background thread");
            }

            Console.WriteLine();

            if (Thread.CurrentThread.IsThreadPoolThread)
            {
                Console.WriteLine("It is a ThreadPoolThread");
            }
            else
            {
                Console.WriteLine("It is a non-ThreadPoolThread");
            }

            string DecodedText = Encoding.UTF8.GetString(buffer, 0, buffer.Length);

            string[] strings = DecodedText.Split('\n');

            for (int index = 0; index < strings.Length;index++)
            {
                Console.WriteLine(strings[index]);
            }
        }

输出：

Main method: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
ProcessReadWriteAsync: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
ReadWriteAsync 1 parameters: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
ReadWriteAsync 2 parameters: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
ProcessReadWriteAsync: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
ReadWriteAsync 1 parameters: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
ReadWriteAsync 2 parameters: The thread executing this task is , 1
And it is a non-background thread

It is a non-ThreadPoolThread
DisplayResult: The thread executing this method is , 4
And it is a background thread.

It is a ThreadPoolThread
1. msdn.microsoft.com/library/hh191443.aspx                83732
2. msdn.microsoft.com/library/aa578028.aspx               205273
3. msdn.microsoft.com/library/jj155761.aspx                29019
4. msdn.microsoft.com/library/hh290140.aspx               117152
5. msdn.microsoft.com/library/hh524395.aspx                68959
6. msdn.microsoft.com/library/ms404677.aspx               197325
7. msdn.microsoft.com                                     42972
8. msdn.microsoft.com/library/ff730837.aspx               146159
9. msdn.microsoft.com/library/hh191443.aspx                83732
10. msdn.microsoft.com/library/aa578028.aspx               205273
11. msdn.microsoft.com/library/jj155761.aspx                29019
12. msdn.microsoft.com/library/hh290140.aspx               117152
13. msdn.microsoft.com/library/hh524395.aspx                68959
14. msdn.microsoft.com/library/ms404677.aspx               197325
15. msdn.microsoft.com                                     42972
16. msdn.microsoft.com/library/ff730837.aspx               146159
17. msdn.microsoft.com/library/hh191443.aspx                83732
18. msdn.microsoft.com/library/aa578028.aspx               205273
19. msdn.microsoft.com/library/jj155761.aspx                29019
20. msdn.microsoft.com/library/hh290140.aspx               117152
21. msdn.microsoft.com/library/hh524395.aspx                68959
22. msdn.microsoft.com/library/ms404677.aspx               197325
23. msdn.microsoft.com                                     42972
24. msdn.microsoft.com/library/ff730837.aspx               146159
25. msdn.microsoft.com/library/hh191443.aspx                83732
26. msdn.microsoft.com/library/aa578028.aspx               205273
27. msdn.microsoft.com/library/jj155761.aspx                29019
28. msdn.microsoft.com/library/hh290140.aspx               117152
29. msdn.microsoft.com/library/hh524395.aspx                68959
30. msdn.microsoft.com/library/ms404677.aspx               197325
31. msdn.microsoft.com                                     42972
32. msdn.microsoft.com/library/ff730837.aspx               146159
33. msdn.microsoft.com/library/hh191443.aspx                83732
34. msdn.microsoft.com/library/aa578028.aspx               205273
35. msdn.microsoft.com/library/jj155761.aspx                29019
36. msdn.microsoft.com/library/hh290140.aspx               117152
37. msdn.microsoft.com/library/hh524395.aspx                68959
38. msdn.microsoft.com/library/ms404677.aspx               197325
39. msdn.microsoft.com                                     42972
40. msdn.microsoft.com/library/ff730837.aspx               146159
41. msdn.microsoft.com/library/hh191443.aspx                83732
42. msdn.microsoft.com/library/aa578028.aspx               205273
43. msdn.microsoft.com/library/jj155761.aspx                29019
44. msdn.microsoft.com/library/hh290140.aspx               117152
45. msdn.microsoft.com/library/hh524395.aspx                68959
46. msdn.microsoft.com/library/ms404677.aspx               197325
47. msdn.microsoft.com                                     42972
48. msdn.microsoft.com/library/ff730837.aspx               146159
49. msdn.microsoft.com/library/hh191443.aspx                83732
50. msdn.microsoft.com/library/aa578028.aspx               205273
51. msdn.microsoft.com/library/jj155761.aspx                29019
52. msdn.microsoft.com/library/hh290140.aspx               117152
53. msdn.microsoft.com/library/hh524395.aspx                68959
54. msdn.microsoft.com/library/ms404677.aspx               197325
55. msdn.microsoft.com                                     42972
56. msdn.microsoft.com/library/ff730837.aspx               146159
57. msdn.microsoft.com/library/hh191443.aspx                83732
58. msdn.microsoft.com/library/aa578028.aspx               205273
59. msdn.microsoft.com/library/jj155761.aspx                29019
60. msdn.microsoft.com/library/hh290140.aspx               117152
61. msdn.microsoft.com/library/hh524395.aspx                68959
62. msdn.microsoft.com/library/ms404677.aspx               197325
63. msdn.microsoft.c

DisplayResult: The thread executing this method is , 5
And it is a background thread.

It is a ThreadPoolThread
1. msdn.microsoft.com/library/hh191443.aspx                83732
2. msdn.microsoft.com/library/aa578028.aspx               205273
3. msdn.microsoft.com/library/jj155761.aspx                29019
4. msdn.microsoft.com/library/hh290140.aspx               117152
5. msdn.microsoft.com/library/hh524395.aspx                68959
6. msdn.microsoft.com/library/ms404677.aspx               197325
7. msdn.microsoft.com                                     42972
8. msdn.microsoft.com/library/ff730837.aspx               146159
9. msdn.microsoft.com/library/hh191443.aspx                83732
10. msdn.microsoft.com/library/aa578028.aspx               205273
11. msdn.microsoft.com/library/jj155761.aspx                29019
12. msdn.microsoft.com/library/hh290140.aspx               117152
13. msdn.microsoft.com/library/hh524395.aspx                68959
14. msdn.microsoft.com/library/ms404677.aspx               197325
15. msdn.microsoft.com                                     42972
16. msdn.microsoft.com/library/ff730837.aspx               146159
17. msdn.microsoft.com/library/hh191443.aspx                83732
18. msdn.microsoft.com/library/aa578028.aspx               205273
19. msdn.microsoft.com/library/jj155761.aspx                29019
20. msdn.microsoft.com/library/hh290140.aspx               117152
21. msdn.microsoft.com/library/hh524395.aspx                68959
22. msdn.microsoft.com/library/ms404677.aspx               197325
23. msdn.microsoft.com                                     42972
24. msdn.microsoft.com/library/ff730837.aspx               146159
25. msdn.microsoft.com/library/hh191443.aspx                83732
26. msdn.microsoft.com/library/aa578028.aspx               205273
27. msdn.microsoft.com/library/jj155761.aspx                29019
28. msdn.microsoft.com/library/hh290140.aspx               117152
29. msdn.microsoft.com/library/hh524395.aspx                68959
30. msdn.microsoft.com/library/ms404677.aspx               197325
31. msdn.microsoft.com                                     42972
32. msdn.microsoft.com/library/ff730837.aspx               146159
33. msdn.microsoft.com/library/hh191443.aspx                83732
34. msdn.microsoft.com/library/aa578028.aspx               205273
35. msdn.microsoft.com/library/jj155761.aspx                29019
36. msdn.microsoft.com/library/hh290140.aspx               117152
37. msdn.microsoft.com/library/hh524395.aspx                68959
38. msdn.microsoft.com/library/ms404677.aspx               197325
39. msdn.microsoft.com                                     42972
40. msdn.microsoft.com/library/ff730837.aspx               146159
41. msdn.microsoft.com/library/hh191443.aspx                83732
42. msdn.microsoft.com/library/aa578028.aspx               205273
43. msdn.microsoft.com/library/jj155761.aspx                29019
44. msdn.microsoft.com/library/hh290140.aspx               117152
45. msdn.microsoft.com/library/hh524395.aspx                68959
46. msdn.microsoft.com/library/ms404677.aspx               197325
47. msdn.microsoft.com                                     42972
48. msdn.microsoft.com/library/ff730837.aspx               146159
49. msdn.microsoft.com/library/hh191443.aspx                83732
50. msdn.microsoft.com/library/aa578028.aspx               205273
51. msdn.microsoft.com/library/jj155761.aspx                29019
52. msdn.microsoft.com/library/hh290140.aspx               117152
53. msdn.microsoft.com/library/hh524395.aspx                68959
54. msdn.microsoft.com/library/ms404677.aspx               197325
55. msdn.microsoft.com                                     42972
56. msdn.microsoft.com/library/ff730837.aspx               146159
57. msdn.microsoft.com/library/hh191443.aspx                83732
58. msdn.microsoft.com/library/aa578028.aspx               205273
59. msdn.microsoft.com/library/jj155761.aspx                29019
60. msdn.microsoft.com/library/hh290140.aspx               117152
61. msdn.microsoft.com/library/hh524395.aspx                68959
62. msdn.microsoft.com/library/ms404677.aspx               197325
63. msdn.microsoft.c

multithreading

asynchronous

async-await

threadpool

回答 1

Stack Overflow用户

回答已采纳

发布于 2020-04-15 17:39:46

所有异步方法都开始在当前线程上运行。在碰到一个对不完整的await起作用的Task之前，没有什么不同的事情发生。但是，无论您向await提供什么信息，都必须在await实际执行任何操作之前返回一个值(一个Task)。

所以让我们来看看到底发生了什么：

Main一直运行到它调用ProcessReadWriteAsync(@"/tmp/temp1Write.txt")
ProcessReadWriteAsync(string)运行，直到它调用ReadWriteAsync(filePath)
ReadWriteAsync(string)运行，直到它调用ReadWriteAsync(path, "")
ReadWriteAsync(string,string)运行，直到它调用stream.ReadAsync(buffer, 0, buffer.Length)
stream.ReadAsync()为止，直到它在ReadWriteAsync(string,string)中返回一个不完整的Task
The await为止，在ReadWriteAsync(string,string)中看到不完整的Task并返回一个新的不完全Task，当该方法的其余部分注册为该Task
ReadWriteAsync(string)的延续时，一个不完整的Task
ProcessReadWriteAsync(string)返回一个不完整的Task
There在Main()中还没有await，所以执行继续there.
Main()调用ProcessReadWriteAsync(@"/tmp/temp2Write.txt")，这将再次启动整个进程。

所有这些都发生在同一个线程上。

当您最后调用await task1时，它告诉它暂停执行，直到Task完成为止。此时，您的代码不再运行。

当stream.ReadAsync()最终完成时，它的Task被设置为Completed，而ReadWriteAsync(string,string)的其余部分一直运行到完成，然后触发ReadWriteAsync(string)运行到完成，然后触发ProcessReadWriteAsync(string)运行到完成。所有这些都发生在后台线程上，这就是为什么您看到DisplayResult在后台线程上运行的原因。

一旦task1设置为完成，您的Main()方法将恢复。

请记住，在具有同步上下文的应用程序(如UI应用程序或ASP.NET (非核心))中，连续性不会发生在后台线程上。它们将发生在与所启动的线程相同的线程上。因此，在这些情况下，延续甚至不会开始，直到您点击await task1。但是，您可以告诉它，您不需要它们返回到与.ConfigureAwait(false)相同的同步上下文。

票数 3

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/61232914

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