需求：当线程池中的任务都完成时，打印“所有任务已完成”

代码：

                bool pool = ThreadPool.SetMaxThreads(9, 9);
                if (pool)
                {
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数1"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数2"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数3"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数4"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数5"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数6"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数7"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数8"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数9"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数10"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数11"));
                };
    
                Console.WriteLine("所有任务已完成");

输出结果：

这样写明显是不行的，要想达到目的，需要用到ManualResetEvent

更改后的代码：

                bool pool = ThreadPool.SetMaxThreads(9, 9);
                ManualResetEvent mre = new ManualResetEvent(false);
                if (pool)
                {
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数1"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数2"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数3"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数4"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数5"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数6"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数7"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数8"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数9"));
                    ThreadPool.QueueUserWorkItem(o => this.DoSomethingLong("参数10"));
                    ThreadPool.QueueUserWorkItem(o => {
                        this.DoSomethingLong("参数11");
                        mre.Set();
                    });

                };
                mre.WaitOne();
                Console.WriteLine("所有任务已完成");

输出结果：

好了，看输出结果，目的达到了

这里主要用到了ManualResetEvent类中的Set()和WaitOne()方法

Set(): 将事件状态设置为有信号，其他线程继续执行。

WaitOne():阻止当前线程，简单来说是等待，等待set()的信号，等有信号后放开当前线程，继续执行，

以上是个人理解总结，如有理解错误，请指出，共同进步