private void button2_Click(object sender, EventArgs e)
        {
            waitCallback = new WaitCallback(MyThreadWork);

            ThreadPool.QueueUserWorkItem(waitCallback, "第一个线程");
            ThreadPool.QueueUserWorkItem(waitCallback, "第二个线程");
            ThreadPool.QueueUserWorkItem(waitCallback, "第三个线程");
            ThreadPool.QueueUserWorkItem(waitCallback, "第四个线程");

        }
        public void MyThreadWork(object state)
        {
            int i=0;
            while (i<20)
            {
                i++;
                SetText(string.Format("线程现在开始启动…… {0}", (string)state));
                Thread.Sleep((new Random(5)).Next()*1000);
                SetText(string.Format("运行结束…… {0}", (string)state));
            }
        }
        public delegate void mydeleg(string str); 
        public void SetText(string str)
        {
            if (textBox1.InvokeRequired)
            {
                textBox1.Invoke(new mydeleg(SetText),str);
            }
            else
                textBox1.Text = str;
        }