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：