先看代码：

                 //设置可以同时处于活动状态的线程池的请求数目。 
                bool pool = ThreadPool.SetMaxThreads(8, 8);
                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"));
                };

上面代码先设置线程池中最大并发量为8个，然后通过QueueUserWorkItem向线程池中添加11个方法，运行，输出结果：

可以看出，先运行了8个，当有一个任务结束后线程池中有空闲线程时，排队的下一个任务才会执行，

把最大并发量改成9试试：

{
                //设置可以同时处于活动状态的线程池的请求数目。 
                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"));
                };
               
            }

运行结果：

果然没错，这次是先执行9个，当有空闲线程时再执行下一个

总结一下

QueueUserWorkItem：将方法排入队列以便执行。 此方法在有线程池线程变得可用时执行。