异步程序未异步运行的奇怪行为

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【中文标题】异步程序未异步运行的奇怪行为【英文标题】：Strange behavior with asynchronous program not running asynchronously 【发布时间】：2020-07-28 16:04:29 【问题描述】：

我在下面的程序中打印了哪个线程执行每个调用的方法，结果很奇怪。我希望所有异步调用都由线程池线程执行，因为@Jon Skeet 提到异步调用使用 ThreadPool 线程并且线程池线程是后台线程。

线程 id 显示执行 Main 方法的同一线程执行所有调用的方法，但 DisplayResult 不是异步方法。

DisplayResult 的每个调用（不是异步方法）是如何由线程池线程执行的，而实际的异步方法在调用时是由非后台线程执行的，与执行主要方法？以及这个程序应该有多少个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

【问题讨论】：

"public static async Task Main()" 更像是在 Main 方法中启用 await 的语法糖，因此它的执行可能与您的预期不同（就像您观察到的代码一样）。 忽略async。忽略任何关于线程的想法。阅读您的代码，就好像它是单线程的并且完全正常，直到您点击 await 并且已评估其右侧的任何内容以生成 Task （包括在右侧输入任何方法调用）它）。恭喜，这正是发生的事情。在您第一次点击await 之前，您在方法调用链中走了多远？ I expected all asynchronous calls to be executed by a threadpool thread because @Jon Skeet mentioned that asynchronous calls use ThreadPool threads 这绝对是错误的。我怀疑 Jon Skeet 实际上是这么说的，因为它不正确，但如果他这样做了，请链接到问题/答案，以便更正。这很简单，如果您不使用 Task.Run() 进行异步调用，那么您就没有使用线程池线程。一切都在一个线程上运行。当然，除非 async 方法不尊重 API 礼仪并调用 Task.Run() 或在实现中产生其他线程。 不要过分强调这一点，@MyWrathAcademia，但我建议您接近 cmets/answers，就好像您从未听说过有关 async/await 的任何信息，而不是主要考虑您理解它，但只需要澄清一些细节。这里有很多误解。 @MyWrathAcademia 因为 I/O 绑定代码存在固有的等待。当您的代码从文件请求字节时，它可以在等待时执行其他操作，例如尝试从另一个文件读取，即使没有第二个线程。这些来自Eric Lippert (an old article about C# 5.0 CTP, but still a good read) 和Steven Cleary 的文章是必读的。 【参考方案1】：

所有异步方法都开始在当前线程上运行。在您点击作用于不完整的Task 的await 之前，不会发生任何不同。但是无论你向await 提供什么，都必须在await 实际执行任何操作之前实际返回一个值（Task）。

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) 一直运行到它调用 ReadWriteAsync(filePath) ReadWriteAsync(string) 一直运行到它调用 ReadWriteAsync(path, "") ReadWriteAsync(string,string) 一直运行到它调用 stream.ReadAsync(buffer, 0, buffer.Length) stream.ReadAsync() 做了一些魔法，直到它返回一个不完整的 Task ReadWriteAsync(string,string) 中的await 看到不完整的Task 并且返回一个新的不完整Task，该方法的其余部分注册为该Task 的延续 ReadWriteAsync(string) 返回一个不完整的 Task ProcessReadWriteAsync(string) 返回一个不完整的 Task Main() 中还没有await，所以在那里继续执行。 Main() 调用ProcessReadWriteAsync(@"/tmp/temp2Write.txt")，这又开始了整个过程。

所有这些都发生在同一个线程上。

当你最终调用await task1 时，它会告诉它暂停执行，直到Task 完成。此时，您的任何代码都不再运行。

当stream.ReadAsync() 最终完成时，它的Task 设置为Completed，ReadWriteAsync(string,string) 的其余部分运行直到完成，然后触发ReadWriteAsync(string) 运行到完成，然后触发ProcessReadWriteAsync(string) 运行完成。所有这些都发生在后台线程上，这就是为什么您会看到 DisplayResult 在后台线程上运行。

一旦task1 设置为完成，您的Main() 方法就会恢复。

请记住，在具有同步上下文的应用程序中，例如 UI 应用程序或 ASP.NET（不是 Core），继续不会发生在后台线程上。它们将发生在他们开始的同一线程上。因此，在这些情况下，直到您点击 await task1 之后，延续才会开始。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。

【讨论】：

Async 现在更清晰了，谢谢。何时返回不完整的Task？是调用返回异步方法的 TaskResult 的瞬间吗？是一个不完整的Task Task 返回一个TaskResult - 这就是为什么只有stream.ReadAsync 返回一个不完整的Task 因为所有其他被调用的Async 方法只返回Task？ 而await 作用于由stream.ReadAsync 返回的不完整的Task 导致通过函数调用堆栈的ReadWriteAsync(string, string)、ReadWriteAsync(string)、ProcessReadWriteAsync(string) 的帧返回（即最后一个先进先出），就像在递归中一样？为什么方法中的其余代码在后台线程上运行完成？这是一个武断的决定，还是有特定原因导致挂起方法中的剩余代码不在非后台线程上运行？ “不完整的任务何时返回？” - 当stream.ReadAsync() 返回一个时。一种真正的异步方法（进行 I/O 操作并暂停代码执行直到完成）实际上返回一个继承自 Task 的对象。最简单的例子是如果您查看the code for Task.Delay() - 它返回一个DelayPromise 对象，该对象继承自Task&lt;&gt;。 “为什么方法中的其余代码会在后台线程上运行完成？” - 因为这样会更快地完成（它不必等待原始线程被释放）并且没有理由不这样做。但是，在 UI 应用程序中，您只能从单个线程修改 UI，因此有理由在它开始的线程上恢复执行，这就是它在那里工作不同的原因。控制台应用程序没有这个限制。 这些链接非常有用。因此，异步方法一直运行到 return 命令返回一个名为 的承诺，并且该承诺代表不完整的 Task?

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