做多线程时访问控件遇到的问题！望高手能解决！

Question

输出窗口总显示  c# 在 System.NullReferenceException 中第一次偶然出现的请教高人有办法帮忙吗

Answer 1

        private static ManualResetEvent ev = new ManualResetEvent(false);   //父、子线程同步           private Thread thread;
       public FrmMain(int _ip, ManualResetEvent _ev)
        {
            ip = _ip; ev = _ev;            thread = new Thread(new ThreadStart(Scan));            lock (typeof(FrmMain)) { instanceCount++; }   //同时存在的对象个数加一  
        }

Answer 2

Scan 在哪儿声明的？instanceCount 在哪儿声明的？这点代码看不出什么来...

Answer 3

        private static int MaxThreadCount = 200;
        public static int instanceCount = 0;     //同时存在的对象个数        public void Open()
        {
            System.Threading.Timer timer1 = new System.Threading.Timer(new TimerCallback(OnTimer), null, 0, 200);  //200
            ev.WaitOne();
        }        private static void OnTimer(object state)
        {
            lock (typeof(FrmMain))
            {                if ((FrmMain.instanceCount < MaxThreadCount) && (startIP <= endIP))
                {
                    while (startIP <= endIP)
                    {
                        FrmMain ck = new FrmMain(startIP, ev);
                        ck.Scan();
                        if (FrmMain.instanceCount >= MaxThreadCount) break;
                        startIP++;
                    }
                }
            }
        }     private void Scan()
        {
            try
            {                string IP = "192.168.1." + this.ip;
                string Return = CPing.PingResult(IP);            }
catch{ }
         }