写了一个例子代码
感觉使用while来扫描不是很好
应该使用同步机制的

using System;
using System.Threading;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    public class Program
    {
        static void Main()
        {
            MyThreadPool threadPool = new MyThreadPool(100, 10);
            threadPool.Start();

            Console.ReadKey();
        }
    }

    /// <summary>
    /// 自定义线程池
    /// </summary>
    public class MyThreadPool
    {
        /// <summary>
        /// 线程队列
        /// </summary>
        private Queue<Thread> _ThreadQueue;

        /// <summary>
        /// 当前线程数
        /// </summary>
        private Int32 _CurThreadCount;

        /// <summary>
        /// 同一时间最大能运行的线程数
        /// </summary>
        private Int32 _MaxRunThreadNumber;

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="maxThreadNumber">最大线程数</param>
        /// <param name="maxRunThreadNumber">同一时间最大能运行的线程数</param>
        public MyThreadPool(Int32 maxThreadNumber,Int32 maxRunThreadNumber)
        {
            _ThreadQueue = new Queue<Thread>(maxThreadNumber);

            if (maxRunThreadNumber > maxThreadNumber)
            {
                _MaxRunThreadNumber = maxThreadNumber;
            }
            else
            {
                _MaxRunThreadNumber = maxRunThreadNumber;
            }

            // 初始化线程队列
            for (Int32 i = 0; i < maxThreadNumber; i++)
            {
                Thread t = new Thread(new ParameterizedThreadStart(Foo));
                t.IsBackground = true;
                t.Name = String.Format("线程{0}", i + 1);
                _ThreadQueue.Enqueue(t);
            }

            _CurThreadCount = 0;
        }

        /// <summary>
        /// 模拟运行工作
        /// </summary>
        /// <param name="threadName"></param>
        private void Foo(Object threadName)
        {
            Random r = new Random(Guid.NewGuid().GetHashCode());
            Int32 iSleep = r.Next(1, 10) * 1000;

            Interlocked.Increment(ref _CurThreadCount);
            Console.WriteLine(String.Format("{0} 启动,延时 {1} 毫秒, 当前运行线程数据 {2}", threadName, iSleep, _CurThreadCount));

            Thread.Sleep(iSleep);

            Interlocked.Decrement(ref _CurThreadCount);

            Console.WriteLine(String.Format("{0} 结束", threadName));
        }

        /// <summary>
        /// 开启线程
        /// </summary>
        public void Start()
        {
            while (true)
            {
                // 如果线程队列为空,则退出
                if (_ThreadQueue.Count == 0)
                {
                    break;
                }

                // 如果当前运行的线程数据小于最大运行数据,就从队列里取一个线程运行
                if (_CurThreadCount < _MaxRunThreadNumber)
                {
                    Thread t = _ThreadQueue.Dequeue();
                    t.Start(t.Name);
                }

                Thread.Sleep(50);
            }
        }
    }
}