生成流水号问题.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace ShareVar
{
    class SerialNum
    {
        private static int _num = 0;
        private object lockKey = new object();
        public SerialNum()
        {
            //_num = 0;
        }

        public int Num
        {
            get
            {
                //thread safe mode by lock 

                lock (lockKey)
                {
                    _num++;
                    int myNum = _num;
                    //System.Threading.Thread.Sleep(100);
                    return myNum;
                }

                //thread safe by readwritelock

                //int myNum;
                //ReaderWriterLock rwl = new ReaderWriterLock();
               
                ////read 
                //rwl.AcquireReaderLock(Timeout.Infinite);
                //myNum = _num + 1;
                //rwl.ReleaseReaderLock();

                //rwl.AcquireWriterLock(Timeout.Infinite);
                //_num = myNum;
                //rwl.ReleaseWriterLock();

                //System.Threading.Thread.Sleep(100);
                //return myNum;


                //thread not safe
                //_num++;
                //int myNum = _num;
                //System.Threading.Thread.Sleep(1000);
                //return myNum;
                
            }
        }

        
       
    }


    class Test
    {
        private SerialNum sn = new SerialNum();
        static void Main(string[] args)
        {
            Thread[] threadArray = new Thread[20];
            int threadNum;
            Test test = new ShareVar.Test();
            ThreadStart myThreadStart = new ThreadStart(test.SendMsg);
            for (threadNum = 0; threadNum < 20; threadNum++)
            {
                threadArray[threadNum] = new Thread(myThreadStart);
            }

            //thread start
            for (threadNum = 0; threadNum < 20; threadNum++)
            {
                threadArray[threadNum].Start();
            }

            ////Wait until all the thread spawn out finish.
            for (threadNum = 0; threadNum < 20; threadNum++)
            {
                threadArray[threadNum].Join();
            }
            Console.ReadLine();

        }

        public void SendMsg()
        {
            int myNum = sn.Num;
            Console.WriteLine(myNum);
        }
    }
}