C#多线程---线程池的工作者线程

一、线程池简介

创建和销毁线程是一个要耗费大量时间的过程，太多的线程也会浪费内存资源，所以通过Thread类来创建过多的线程反而有损于性能，为了改善这样的问题，.net中就引入了线程池。

线程池形象的表示就是存放应用程序中使用的线程的一个集合。

CLR初始化时，线程池中是没有线程的，在内部，线程池维护了一个操作请求队列，当应用程序想执行一个异步操作时，就调用一个方法，就将一个任务放到线程池的队列中，线程池中代码从队列中提取任务，将这个任务委派给一个线程池线程去执行，当线程池线程完成任务时，线程不会被销毁，而是返回到线程池中，等待响应另一个请求。由于线程不被销毁，这样就可以避免因为创建线程所产生的性能损失。

注意：通过线程池创建的线程默认为后台线程，优先级默认为Normal.

二、创建方法

public static bool QueueUserWorkItem (WaitCallback callBack);

public static bool QueueUserWorkItem(WaitCallback callback, Object state);

这两个方法向线程池的队列添加一个工作项（work item）以及一个可选的状态数据。

工作项其实就是由callback参数标识的一个方法，该方法将由线程池线程执行。

回调方法必须匹配System.Threading.WaitCallback委托类型，定义为：

public delegate void WaitCallback(Object state);

三、使用例子

（1）正常使用

 1 using System;

 2 using System.Threading;

 3

 4 namespace ThreadPoolWorkerItem

 5 {

 6     class Program

 7     {

 8         static void Main(string[] args)

 9         {

10             Console.WriteLine("Thread pool thread test...");

11             Console.WriteLine();

12             string myInfo = "The following is the thread info ontained in function Main";

13             PrintThreadInfo(myInfo);

14             Console.WriteLine();

15             ThreadPool.QueueUserWorkItem(AsyncMethod);

16             Console.ReadLine();

17

18         }

19         private static void PrintThreadInfo(string info)

20         {

21             Console.WriteLine(info);

22             Console.WriteLine($"ThreadId:{Thread.CurrentThread.ManagedThreadId}\nIsBackgroundThread:{Thread.CurrentThread.IsBackground}\nIsThreadPoolThread:{Thread.CurrentThread.IsThreadPoolThread}");

23             int workerThread = 0;

24             int ioThread = 0;

25             ThreadPool.GetMaxThreads(out workerThread, out ioThread);

26             Console.WriteLine($"MaxWorkerThread:{workerThread}\nMaxIoThread:{ioThread}");

27             int workerThreadAvailable = 0;

28             int ioThreadAvailable = 0;

29             ThreadPool.GetAvailableThreads(out workerThreadAvailable, out ioThreadAvailable);

30             Console.WriteLine($"AvailableWorkerThread:{workerThreadAvailable}\nAvailableIoThread:{ioThreadAvailable}");

31         }

32         private  static void AsyncMethod(object state)

33         {

34             string myInfo = "The following is the thread info ontained in function AsyncMethod";

35             PrintThreadInfo(myInfo);

36             Thread.Sleep(3000);

37             Console.WriteLine("AsyncMethod done!");

38         }

39     }

40 }

运行结果如下：

（2）任务取消

　　下面使用System.Threading.CancellationTokenSource对象来对任务进行取消操作。　

 1 using System;

 2 using System.Threading;

 3

 4 namespace ThreadPoolWorkerItem

 5 {

 6     class Program

 7     {

 8         static void Main(string[] args)

 9         {

10             Console.WriteLine("Thread pool thread test...");

11             Console.WriteLine();

12             //string myInfo = "The following is the thread info ontained in function Main";

13             //PrintThreadInfo(myInfo);

14             //Console.WriteLine();

15

16             CancellationTokenSource cts = new CancellationTokenSource();

17             ThreadPool.QueueUserWorkItem(AsyncMethod, cts.Token);

18

19             while(true)

20             {

21                 string myLine = Console.ReadLine();

22                 if(myLine == "quit")

23                 {

24                     break;

25                 }

26                 switch(myLine)

27                 {

28                     case "cancel":

29                         Console.WriteLine("Cancel request from user...");

30                         cts.Cancel();

31                         break;

32                 }

33             }

34

35         }

36         private static void PrintThreadInfo(string info)

37         {

38             Console.WriteLine(info);

39             Console.WriteLine($"ThreadId:{Thread.CurrentThread.ManagedThreadId}\nIsBackgroundThread:{Thread.CurrentThread.IsBackground}\nIsThreadPoolThread:{Thread.CurrentThread.IsThreadPoolThread}");

40             int workerThread = 0;

41             int ioThread = 0;

42             ThreadPool.GetMaxThreads(out workerThread, out ioThread);

43             Console.WriteLine($"MaxWorkerThread:{workerThread}\nMaxIoThread:{ioThread}");

44             int workerThreadAvailable = 0;

45             int ioThreadAvailable = 0;

46             ThreadPool.GetAvailableThreads(out workerThreadAvailable, out ioThreadAvailable);

47             Console.WriteLine($"AvailableWorkerThread:{workerThreadAvailable}\nAvailableIoThread:{ioThreadAvailable}");

48         }

49         private  static void AsyncMethod(object state)

50         {

51             //string myInfo = "The following is the thread info ontained in function AsyncMethod";

52             //PrintThreadInfo(myInfo);

53             CancellationToken cltToken = (CancellationToken)state;

54             int myCountNumber = 100;

55             for (int i = 0; i < myCountNumber; i++)

56             {

57                 Console.WriteLine(i);

58                 if (cltToken.IsCancellationRequested)

59                 {

60                     Console.ForegroundColor = ConsoleColor.DarkRed;

61                     Console.WriteLine($"Counting is cancelled. Current:{i} / Total:{myCountNumber}");

62                     Console.ResetColor();

63                     return;

64                 }

65                 Thread.Sleep(3000);

66

67             }

68             Console.WriteLine("Counting is done!");

69         }

70     }

71 }

运行结果如下：

巴特西

C#多线程---线程池的工作者线程

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