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

using System;
using System.Threading;

namespace ThreadPoolWorkerItem
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Thread pool thread test...");
            Console.WriteLine();
            string myInfo = "The following is the thread info ontained in function Main";
            PrintThreadInfo(myInfo);
            Console.WriteLine();
            ThreadPool.QueueUserWorkItem(AsyncMethod);
            Console.ReadLine();

        }
        private static void PrintThreadInfo(string info)
        {
            Console.WriteLine(info);
            Console.WriteLine($"ThreadId:{Thread.CurrentThread.ManagedThreadId}
IsBackgroundThread:{Thread.CurrentThread.IsBackground}
IsThreadPoolThread:{Thread.CurrentThread.IsThreadPoolThread}");
            int workerThread = 0;
            int ioThread = 0;
            ThreadPool.GetMaxThreads(out workerThread, out ioThread);
            Console.WriteLine($"MaxWorkerThread:{workerThread}
MaxIoThread:{ioThread}");
            int workerThreadAvailable = 0;
            int ioThreadAvailable = 0;
            ThreadPool.GetAvailableThreads(out workerThreadAvailable, out ioThreadAvailable);
            Console.WriteLine($"AvailableWorkerThread:{workerThreadAvailable}
AvailableIoThread:{ioThreadAvailable}");
        }
        private  static void AsyncMethod(object state)
        {
            string myInfo = "The following is the thread info ontained in function AsyncMethod";
            PrintThreadInfo(myInfo);
            Thread.Sleep(3000);
            Console.WriteLine("AsyncMethod done!");
        }
    }
}

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。　

using System;
using System.Threading;

namespace ThreadPoolWorkerItem
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Thread pool thread test...");
            Console.WriteLine();
            //string myInfo = "The following is the thread info ontained in function Main";
            //PrintThreadInfo(myInfo);
            //Console.WriteLine();

            CancellationTokenSource cts = new CancellationTokenSource();
            ThreadPool.QueueUserWorkItem(AsyncMethod, cts.Token);

            while(true)
            {
                string myLine = Console.ReadLine();
                if(myLine == "quit")
                {
                    break;
                }
                switch(myLine)
                {
                    case "cancel":
                        Console.WriteLine("Cancel request from user...");
                        cts.Cancel();
                        break;                       
                }
            }

        }
        private static void PrintThreadInfo(string info)
        {
            Console.WriteLine(info);
            Console.WriteLine($"ThreadId:{Thread.CurrentThread.ManagedThreadId}
IsBackgroundThread:{Thread.CurrentThread.IsBackground}
IsThreadPoolThread:{Thread.CurrentThread.IsThreadPoolThread}");
            int workerThread = 0;
            int ioThread = 0;
            ThreadPool.GetMaxThreads(out workerThread, out ioThread);
            Console.WriteLine($"MaxWorkerThread:{workerThread}
MaxIoThread:{ioThread}");
            int workerThreadAvailable = 0;
            int ioThreadAvailable = 0;
            ThreadPool.GetAvailableThreads(out workerThreadAvailable, out ioThreadAvailable);
            Console.WriteLine($"AvailableWorkerThread:{workerThreadAvailable}
AvailableIoThread:{ioThreadAvailable}");
        }
        private  static void AsyncMethod(object state)
        {
            //string myInfo = "The following is the thread info ontained in function AsyncMethod";
            //PrintThreadInfo(myInfo);
            CancellationToken cltToken = (CancellationToken)state;
            int myCountNumber = 100;
            for (int i = 0; i < myCountNumber; i++)
            {
                Console.WriteLine(i);
                if (cltToken.IsCancellationRequested)
                {
                    Console.ForegroundColor = ConsoleColor.DarkRed;
                    Console.WriteLine($"Counting is cancelled. Current:{i} / Total:{myCountNumber}");
                    Console.ResetColor();
                    return;
                }
                Thread.Sleep(3000);
            
            }
            Console.WriteLine("Counting is done!");
        }
    }
}

运行结果如下：