使用HttpClient的异步文件下载时，线程问题 [英] Threading issues when using HttpClient for asynchronous file downloads

查看：176 发布时间：2016/7/20 11:43:30 c# asynchronous download async-await dotnet-httpclient

本文介绍了使用HttpClient的异步文件下载时，线程问题的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

2015/01/15编辑在住宿增加对多线程 - 仍然有一个谜，

 使用系统;
使用System.Collections.Generic;
使用System.IO;
使用System.Net.Http;
使用System.Threading.Tasks;命名空间TestHttpClient2
{
  类节目
  {
    / *使用雅虎门户网站访问报价的股票 - 执行异步操作。 * /    静态字符串的baseUrl =http://real-chart.finance.yahoo.com/;
    静态字符串requestUrlFormat =/table.csv?s={0}&d=0&e=1&f=2016&g=d&a=0&b=1&c=1901&ignore=.csv ;    静态无效的主要（字串[] args）
    {
      VAR activeTaskList =新的List＆LT;任务＆GT;（）;      字符串输出目录=StockQuotes;
      如果（！Directory.Exists（输出目录））
      {
        Directory.CreateDirectory（输出目录）;
      }      而（真）
      {
        Console.WriteLine（输入符号或回车键退出：）;
        串符号=到Console.ReadLine（）;
        如果（string.IsNullOrEmpty（符号））
        {
          打破;
        }        任务downloadTask = DownloadDataForStockAsync（输出目录，符号）;
        如果（TaskIsActive（downloadTask））
        {
          //这是一个异步的世界 - 更新前锁定列表！
          锁定（activeTaskList）
          {
            activeTaskList.Add（downloadTask）;
          }        }
        其他
        {
          Console.WriteLine（任务已经完成??？！？）;
        }
        CleanupTasks（activeTaskList）;
      }      Console.WriteLine（清理）;
      而（CleanupTasks（activeTaskList））
      {
        Task.Delay（1）.Wait（）;
      }
    }    私人静态布尔CleanupTasks（列表＆LT;任务＆GT; activeTaskList）
    {
      //反向环路允许列表项删除
      //这是一个异步的世界 - 更新前锁定列表！
      锁定（activeTaskList）
      {
        的for（int i = activeTaskList.Count  -  1; I＆GT; = 0;我 - ）
        {
          如果（！TaskIsActive（activeTaskList [I]））
          {
            activeTaskList.RemoveAt（ⅰ）;
          }
        }
        返回activeTaskList.Count＆GT; 0;
      }
    }    私人静态布尔TaskIsActive（任务任务）
    {
      返回的任务！= NULL
          ＆功放;＆安培; task.Status！= TaskStatus.Canceled
          ＆功放;＆安培; task.Status！= TaskStatus.Faulted
          ＆功放;＆安培; task.Status = TaskStatus.RanToCompletion！;
    }    静态异步任务DownloadDataForStockAsync（字符串输出目录，串符号）
    {
      尝试
      {
        使用（VAR的客户=新的HttpClient（））
        {
          client.BaseAddress =新的URI（的baseUrl）;
          client.Timeout = TimeSpan.FromMinutes（5）;
          字符串requestUrl =的String.Format（requestUrlFormat，符号）;          VAR要求=新的Htt prequestMessage（HttpMethod.Post，requestUrl）;
          VAR sendTask = client.SendAsync（请求，HttpCompletionOption.ResponseHeadersRead）;
          VAR响应=等待sendTask;
          response.EnsureSuccessStatus code（）;
          VAR httpStream =等待response.Content.ReadAsStreamAsync（）;          字符串timestampedName = FormatTimestampedString（符号，真正的）;
          VAR文件路径= Path.Combine（输出目录，timestampedName +名为.csv）;
          使用（VAR FILESTREAM = File.Create（文件路径））
          使用（VAR读者=新的StreamReader（httpStream））
          {
            等待httpStream.CopyToAsync（FILESTREAM）;
            fileStream.Flush（）;
          }
        }
      }
      赶上（异常前）
      {
        Console.WriteLine（上线异常：{0}：{1} \\ r \\ n，
          System.Threading.Thread.CurrentThread.ManagedThreadId，
          ex.Message，
          ex.StackTrace）;
      }
    }    静态挥发字符串lastTimestampedString =的String.Empty;
    静态挥发串假=的String.Empty;
    静态的HashSet＆LT;串GT; oldStrings =新的HashSet＆LT;串GT;（）;    静态字符串FormatTimestampedString（字符串消息，布尔uniquify = FALSE）
    {
      //这是一个异步的世界 - 在使用之前锁定共享资源！
      //锁（虚设）
      锁定（lastTimestampedString）
      {
        Console.WriteLine（IN  - 主题：{0：D2} lastTimestampedString：{1}，
            System.Threading.Thread.CurrentThread.ManagedThreadId，
            lastTimestampedString）;        串newTimestampedString;        而（真）
        {
          日期时间lastDateTime = DateTime.Now;          newTimestampedString =的String.format（
              {1：D4} _ {2} D2 _ {3} D2 _ {4} D2 _ {5} D2 _ {6} D2 _ {7：D3} _ {0}，
                信息，
                lastDateTime.Year，lastDateTime.Month，lastDateTime.Day，
                lastDateTime.Hour，lastDateTime.Minute，lastDateTime.Second，
                lastDateTime.Millisecond
                ）;
          如果（！uniquify）
          {
            打破;
          }
          如果（newTimestampedString！= lastTimestampedString）
          {
            打破;
          }          //Task.Delay（1）.Wait（）;
        };        lastTimestampedString = newTimestampedString;
        Console.WriteLine（OUT  - 主题：{0：D2} lastTimestampedString：{1}，
            System.Threading.Thread.CurrentThread.ManagedThreadId，
            lastTimestampedString）;        如果（uniquify）
        {
          oldStrings.Add（lastTimestampedString）;
        }
        返回lastTimestampedString;
      }
    }
  }
}

Q）为什么会出现这种间歇性的错误（在此输出的末端。）（我抄袭NES重复行到剪贴板，并粘贴到控制台的一个长长的清单，以重复的问题）

 输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
NES
输入符号或回车键退出：
IN  - 主题：18 lastTimestampedString：
OUT  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_472_NES
IN  - 主题：17 lastTimestampedString：2015_01_15_11_19_44_472_NES
OUT  - 主题：17 lastTimestampedString：2015_01_15_11_19_44_473_NES
IN  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_473_NES
OUT  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_493_NES
IN  - 主题：16 lastTimestampedString：2015_01_15_11_19_44_493_NES
OUT  - 主题：16 lastTimestampedString：2015_01_15_11_19_44_494_NES
IN  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_494_NES
OUT  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_495_NES
IN  - 主题：17 lastTimestampedString：2015_01_15_11_19_44_495_NES
IN  - 主题：16 lastTimestampedString：2015_01_15_11_19_44_495_NES
OUT  - 主题：17 lastTimestampedString：2015_01_15_11_19_44_496_NES
IN  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_495_NES
OUT  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_496_NES
IN  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_496_NES
OUT  - 主题：16 lastTimestampedString：2015_01_15_11_19_44_495_NES
OUT  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_497_NES
IN  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_497_NES
OUT  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_523_NES
IN  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_523_NES
OUT  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_532_NES
IN  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_532_NES
OUT  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_533_NES
IN  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_533_NES
在线程异常：17：该进程无法访问该文件'C：\\用户\\ drogers \\ _ code \\测试\\ TestHttpClient \\ TestHttpClient2 \\ BIN \\调试\\ StockQuot
ES \\ 2015_01_15_11_19_44_495_NES.csv'，因为它正由另一个进程使用。上线异常：16：该进程无法访问该文件'C：\\用户\\ drogers \\ _ code \\测试\\ TestHttpClient \\ TestHttpClient2 \\ BIN \\调试\\ StockQuot
ES \\ 2015_01_15_11_19_44_496_NES.csv'，因为它正由另一个进程使用。OUT  - 主题：18 lastTimestampedString：2015_01_15_11_19_44_540_NES
IN  - 主题：17 lastTimestampedString：2015_01_15_11_19_44_540_NES
IN  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_540_NES
OUT  - 主题：17 lastTimestampedString：2015_01_15_11_19_44_557_NES
OUT  - 主题：19 lastTimestampedString：2015_01_15_11_19_44_560_NES
在线程异常：19：该进程无法访问该文件'C：\\用户\\ drogers \\ _ code \\测试\\ TestHttpClient \\ TestHttpClient2 \\ BIN \\调试\\ StockQuot
ES \\ 2015_01_15_11_19_44_560_NES.csv'，因为它正由另一个进程使用。

我能避免与线126的反注释的问题和评论127行，如：

  //这是一个异步的世界 - 在使用之前锁定共享资源！
  锁（虚设）
  //锁（lastTimestampedString）

综观IL，用于FormatTimestampedString产生在code中的唯一差别是

  ldsfld串modreq（[mscorlib程序] System.Runtime.CompilerServices.IsVolatile）
            TestHttpClient2.Program :: ** ** lastTimestampedString

与

  ldsfld串modreq（[mscorlib程序] System.Runtime.CompilerServices.IsVolatile）
            TestHttpClient2.Program :: ** **假人

解决方案

字符串是不可改变的。所以，当我把我锁在lastTimestampedString引用，然后改变了它，我不再有我以为我有锁。锁是在老的字符串。任何其他沿线未来将是对新的字符串测试锁，并会因此被允许的。

认错。

This question is a follow-up to Using HttpClient for Asynchronous File downloads.

2015/01/15 Edited to add in accomodation for multithreading - still have a mystery,

using System;
using System.Collections.Generic;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

namespace TestHttpClient2
{
  class Program
  {
    /* Use Yahoo portal to access quotes for stocks - perform asynchronous operations. */

    static string baseUrl = "http://real-chart.finance.yahoo.com/";
    static string requestUrlFormat = "/table.csv?s={0}&d=0&e=1&f=2016&g=d&a=0&b=1&c=1901&ignore=.csv";

    static void Main(string[] args)
    {
      var activeTaskList = new List<Task>();

      string outputDirectory = "StockQuotes";
      if (!Directory.Exists(outputDirectory))
      {
        Directory.CreateDirectory(outputDirectory);
      }

      while (true)
      {
        Console.WriteLine("Enter symbol or [ENTER] to exit:");
        string symbol = Console.ReadLine();
        if (string.IsNullOrEmpty(symbol))
        {
          break;
        }

        Task downloadTask = DownloadDataForStockAsync(outputDirectory, symbol);
        if (TaskIsActive(downloadTask))
        {
          // This is an asynchronous world - lock the list before updating it!
          lock (activeTaskList)
          {
            activeTaskList.Add(downloadTask);
          }

        }
        else
        {
          Console.WriteLine("task completed already?!??!?");
        }
        CleanupTasks(activeTaskList);
      }

      Console.WriteLine("Cleaning up");
      while (CleanupTasks(activeTaskList))
      {
        Task.Delay(1).Wait();
      }
    }

    private static bool CleanupTasks(List<Task> activeTaskList)
    {
      // reverse loop to allow list item deletions
      // This is an asynchronous world - lock the list before updating it!
      lock (activeTaskList)
      {
        for (int i = activeTaskList.Count - 1; i >= 0; i--)
        {
          if (!TaskIsActive(activeTaskList[i]))
          {
            activeTaskList.RemoveAt(i);
          }
        }
        return activeTaskList.Count > 0;
      }
    }

    private static bool TaskIsActive(Task task)
    {
      return task != null
          && task.Status != TaskStatus.Canceled
          && task.Status != TaskStatus.Faulted
          && task.Status != TaskStatus.RanToCompletion;
    }

    static async Task DownloadDataForStockAsync(string outputDirectory, string symbol)
    {
      try
      {
        using (var client = new HttpClient())
        {
          client.BaseAddress = new Uri(baseUrl);
          client.Timeout = TimeSpan.FromMinutes(5);
          string requestUrl = string.Format(requestUrlFormat, symbol);

          var request = new HttpRequestMessage(HttpMethod.Post, requestUrl);
          var sendTask = client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);
          var response = await sendTask;
          response.EnsureSuccessStatusCode();
          var httpStream = await response.Content.ReadAsStreamAsync();

          string timestampedName = FormatTimestampedString(symbol, true);
          var filePath = Path.Combine(outputDirectory, timestampedName + ".csv");
          using (var fileStream = File.Create(filePath))
          using (var reader = new StreamReader(httpStream))
          {
            await httpStream.CopyToAsync(fileStream);
            fileStream.Flush();
          }
        }
      }
      catch (Exception ex)
      {
        Console.WriteLine("Exception on thread: {0}: {1}\r\n",
          System.Threading.Thread.CurrentThread.ManagedThreadId,
          ex.Message,
          ex.StackTrace);
      }
    }

    static volatile string lastTimestampedString = string.Empty;
    static volatile string dummy = string.Empty;
    static HashSet<string> oldStrings = new HashSet<string>();

    static string FormatTimestampedString(string message, bool uniquify = false)
    {
      // This is an asynchronous world - lock the shared resource before using it!
      //lock (dummy)
      lock (lastTimestampedString)
      {
        Console.WriteLine("IN  - Thread: {0:D2} lastTimestampedString: {1}", 
            System.Threading.Thread.CurrentThread.ManagedThreadId,
            lastTimestampedString);

        string newTimestampedString;

        while (true)
        {
          DateTime lastDateTime = DateTime.Now;

          newTimestampedString = string.Format(
              "{1:D4}_{2:D2}_{3:D2}_{4:D2}_{5:D2}_{6:D2}_{7:D3}_{0}",
                message,
                lastDateTime.Year, lastDateTime.Month, lastDateTime.Day,
                lastDateTime.Hour, lastDateTime.Minute, lastDateTime.Second,
                lastDateTime.Millisecond
                );
          if (!uniquify)
          {
            break;
          }
          if (newTimestampedString != lastTimestampedString)
          {
            break;
          }

          //Task.Delay(1).Wait();
        };

        lastTimestampedString = newTimestampedString;
        Console.WriteLine("OUT - Thread: {0:D2} lastTimestampedString: {1}",
            System.Threading.Thread.CurrentThread.ManagedThreadId,
            lastTimestampedString);

        if (uniquify)
        {
          oldStrings.Add(lastTimestampedString);
        }
        return lastTimestampedString;
      }
    }
  }
}

Q) Why am I getting this intermittent error (at the end of this output.) (I am copying a long list of repeated lines of "NES" to the clipboard and pasting into the console in order to duplicate the problem):

Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
NES
Enter symbol or [ENTER] to exit:
IN  - Thread: 18 lastTimestampedString:
OUT - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_472_NES
IN  - Thread: 17 lastTimestampedString: 2015_01_15_11_19_44_472_NES
OUT - Thread: 17 lastTimestampedString: 2015_01_15_11_19_44_473_NES
IN  - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_473_NES
OUT - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_493_NES
IN  - Thread: 16 lastTimestampedString: 2015_01_15_11_19_44_493_NES
OUT - Thread: 16 lastTimestampedString: 2015_01_15_11_19_44_494_NES
IN  - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_494_NES
OUT - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_495_NES
IN  - Thread: 17 lastTimestampedString: 2015_01_15_11_19_44_495_NES
IN  - Thread: 16 lastTimestampedString: 2015_01_15_11_19_44_495_NES
OUT - Thread: 17 lastTimestampedString: 2015_01_15_11_19_44_496_NES
IN  - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_495_NES
OUT - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_496_NES
IN  - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_496_NES
OUT - Thread: 16 lastTimestampedString: 2015_01_15_11_19_44_495_NES
OUT - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_497_NES
IN  - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_497_NES
OUT - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_523_NES
IN  - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_523_NES
OUT - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_532_NES
IN  - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_532_NES
OUT - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_533_NES
IN  - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_533_NES
Exception on thread: 17: The process cannot access the file 'C:\Users\drogers\_code\Tests\TestHttpClient\TestHttpClient2\bin\Debug\StockQuot
es\2015_01_15_11_19_44_495_NES.csv' because it is being used by another process.

Exception on thread: 16: The process cannot access the file 'C:\Users\drogers\_code\Tests\TestHttpClient\TestHttpClient2\bin\Debug\StockQuot
es\2015_01_15_11_19_44_496_NES.csv' because it is being used by another process.

OUT - Thread: 18 lastTimestampedString: 2015_01_15_11_19_44_540_NES
IN  - Thread: 17 lastTimestampedString: 2015_01_15_11_19_44_540_NES
IN  - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_540_NES
OUT - Thread: 17 lastTimestampedString: 2015_01_15_11_19_44_557_NES
OUT - Thread: 19 lastTimestampedString: 2015_01_15_11_19_44_560_NES
Exception on thread: 19: The process cannot access the file 'C:\Users\drogers\_code\Tests\TestHttpClient\TestHttpClient2\bin\Debug\StockQuot
es\2015_01_15_11_19_44_560_NES.csv' because it is being used by another process.

I can avoid the problem with the uncommenting of line 126 and commenting line 127, as in:

  // This is an asynchronous world - lock the shared resource before using it!
  lock (dummy)
  //lock (lastTimestampedString)

Looking at the il, the only difference in the code generated for FormatTimestampedString is

ldsfld     string modreq([mscorlib]System.Runtime.CompilerServices.IsVolatile)
            TestHttpClient2.Program::**lastTimestampedString**

versus

ldsfld     string modreq([mscorlib]System.Runtime.CompilerServices.IsVolatile) 
            TestHttpClient2.Program::**dummy**

解决方案

Strings are immutable. So when I set my lock on the lastTimestampedString reference, and then changed it, I no longer had the lock I thought I had. The lock was on the old string. Anyone else coming along would be testing the lock on the new string, and would therefore be allowed in.

Mea culpa.

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