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：