ThreadPool.SetMaxThreads（5,5）

表示活动线程的数量为5（如果你有超过5个CPU核心），不并不意味着线程池只能创建5个线程。
线程= CPU核心* 250。

在 Thread.sleep代码线程池的最大数量，线程是不活动的，所以它不会影响到其他的线程的执行。

If my understanding of the way the ThreadPool works is correct, one of its purposes is to limit the number of worker threads within a process that can be created at a given time. For example, if you set MaxThreads to 5 and then call QueueUserWorkItem 30 times, 30 requests will be made to the ThreadPool, but only 5 of those requests will be serviced by a new thread, while the other 25 requests will be added to the queue and serviced one at time as previous requests complete and existing threads becomes available.

In the code below, however, the call to Thread.Sleep(-1) guarantees that the DoSomething() method will never return, meaning that the current thread will never become available to subsequent requests.

But my understanding of the way a ThreadPool works cannot be correct, because if it were correct the code below would print only the numbers 0-4, rather than 0-29.

Can someone please explain how the ThreadPool works and why the below code isn't doing what I thought it should be doing?

    static void DoSomething(object n)
    {
        Console.WriteLine(n);
        Thread.Sleep(-1);
    }

    static void Main(string[] args)
    {
        ThreadPool.SetMaxThreads(5, 5);
        for (int x = 0; x < 30; x++)
        {
            ThreadPool.QueueUserWorkItem(new WaitCallback(DoSomething), x);
        }
        Console.Read();
    }

ThreadPool.SetMaxThreads(5, 5)

means the number of active thread is 5 (if you have more than 5 cpu core), does not mean that the ThreadPool can only create 5 threads. The ThreadPool maximum number of threads = CPU Core * 250.

After Thread.Sleep, the thread is inactive, so it will not affect the execution of other threads.