线程池

　　线程的创建需要时间。如果存在多个任务需要完成，则可以事先创建许多线程，在应该完成任务时发出需求线程请求。线程数最好时动态的增加于减少。TheadPool类可以托管线程列表，这个类能动态增加于减少线程的线程数，直到最大线程数----可配置。在四核CPU中，默认为1023个工作线程和1000个I/O线程。也可以指定创建线程池时应该立即启动的最小线程数，以及可用最大线程数。如果有新的任务，但是线程池中已没有闲于线程，那么新任务需要排队，等待线程完成其任务。如下示例：

　　 

static void Main()
{
    int nWorkerThreads;
    int nCompletionPortThreads;
    ThreadPool.GetMaxThreads(out nWorkerThreads, out nCompletionPortThreads);
    Console.WriteLine("Max worker threads: {0}, I/O completion threads: {1}", nWorkerThreads, nCompletionPortThreads

    for (int i = 0; i < 10; i++)
    {
        ThreadPool.QueueUserWorkItem(JobForAThread); //WaitCallback类型委托：线程池线程要执行的回调方法
    }
    Thread.Sleep(3000);

    /*******************输出结果*****************************
     *           Max worker threads: 1023, I/O completion threads: 1000
     *           loop 0, running inside pooled thread 3
     *           loop 0, running inside pooled thread 4
     *           loop 0, running inside pooled thread 7
     *           loop 0, running inside pooled thread 6
     *           loop 0, running inside pooled thread 8
     *           loop 0, running inside pooled thread 9
     *           loop 0, running inside pooled thread 10
     *           loop 0, running inside pooled thread 5
     *           loop 1, running inside pooled thread 6
     *           loop 1, running inside pooled thread 4
     *           loop 1, running inside pooled thread 7
     *           loop 1, running inside pooled thread 3
     *           loop 1, running inside pooled thread 5
     *           loop 1, running inside pooled thread 8
     *           loop 1, running inside pooled thread 10
     *           loop 1, running inside pooled thread 9
     *           loop 2, running inside pooled thread 6
     *           loop 2, running inside pooled thread 4
     *           loop 2, running inside pooled thread 5
     *           loop 2, running inside pooled thread 7
     *           loop 2, running inside pooled thread 3
     *           loop 2, running inside pooled thread 8
     *           loop 2, running inside pooled thread 10
     *           loop 2, running inside pooled thread 9
     *           loop 0, running inside pooled thread 6
     *           loop 0, running inside pooled thread 4
     *           loop 1, running inside pooled thread 6
     *           loop 1, running inside pooled thread 4
     *           loop 2, running inside pooled thread 6
     *           loop 2, running inside pooled thread 4
     * *******************************************************/
}

static void JobForAThread(object state)
{
    for (int i = 0; i < 3; i++)
    {
        Console.WriteLine("loop {0}, running inside pooled thread {1}", i, Thread.CurrentThread.ManagedThreadId);
        Thread.Sleep(50);
    }
}

　　10个任务，只由8个线程池中的线程处理（因为CPU是8核）。线程池虽然简单，但是有如下限制：

• 线程池中所有线程都是后台线程，且不能将线程池中的线程改为前台线程。如果进程的所有前台线程都结束了，那么所有的后台线程就会停止。
• 不能给入池的线程设置优先级或者线程名称。
• 对于COM对象，入池的所有线程都是多线程单元（MTA）线程。许多COM对象都需要单线程单元（STA）线程。
• 入池的线程只能用于时间较短的任务。如果线程要一直运行，就应该使用Thread类创建一个线程，或者创建Task时使用LongRunning选项。