老师您好：
我想用串口接收和发送数据
我把串口的2.3短接。
我想做个循环，不断地向串口发送和接收数据，就用了个for语句，总是报以下的错
总是报错：未处理unauthorizeaccessexception
          对端口COM1的访问被拒绝
我要是吧for循环去掉，程序运转正常，能发送和接收数据
请老师帮忙，看看那里的错
程序如下：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;
using System.Threading;

namespace test
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            Control.CheckForIllegalCrossThreadCalls = false;

        }
        /// <summary>
        /// 开启接收数据线程
        /// </summary>
        private void ReceiveData(SerialPort serialPort)
        {
             //可用异步接收数据线程
            Thread threadReceiveSub = new Thread(new ParameterizedThreadStart(AsyReceiveData));
            threadReceiveSub.Start(serialPort);
            
        }


        
        

        //异步读取
        private void AsyReceiveData(object serialPortobj)
        {
            SerialPort serialPort = (SerialPort)serialPortobj;
            System.Threading.Thread.Sleep(500);
            try
            {
                 //txtReceive.Text =   serialPort.ReadExisting();
                int bytes = serialPort.BytesToRead;
                byte[] combuffer = new byte[bytes];
                serialPort.Read(combuffer, 0, bytes);

                string str = BitConverter.ToString(combuffer);
                txtReceive.Text = str + txtReceive.Text;
                //字串中是否存在指定字符
                if (str.Contains("63") && str.Contains("5B"))
                {
                    textBox1.Text = "存在";
                }
                else
                {
                    textBox1.Text = "不存在";          
                }
                //查找字符串在字串中的索引
                int m = str.IndexOf("5B");
                txtSend.Text = m.ToString();

                //截取字符串的一部分
                string s1 = str.Substring(0+m, 2);
                textBox2.Text = s1;
                string s2=str.Substring(3+m,2);

                //字符串相加显示
                string s23 = s2 + s1;
                textBox3.Text = s23;
                textBox4.Text = s1 + s2;

                //获取当前年月日
                DateTime currentTime = DateTime.Now;
                string strYMD = currentTime.ToString("d");
                textBox5.Text =strYMD;

                //比较两个字符串，返回值为-1,0,1
                int result=String.Compare(s2,s1);
                textBox6.Text = result.ToString();
            }
            catch (Exception e)
            {
                MessageBox.Show(e.Message);
                //处理错误
            }
            serialPort.Close();
        }
       
        //想串口发送并接收
        private void button1_Click(object sender, EventArgs e)
        {
            for (int i = 0; i < 2; i++)
            {
                System.Threading.Thread.Sleep(1000);   //1   second
                //实例化串口对象(默认：COMM1,9600,e,8,1)            
                SerialPort serialPort1 = new SerialPort();
                //更改参数
                serialPort1.PortName = "COM1";
                serialPort1.BaudRate = 2400;
                //serialPort1.bytesize = 8;
                serialPort1.Parity = Parity.Even;
                serialPort1.StopBits = StopBits.One;
                //try { serialPort1.Open(); }
                //catch { MessageBox.Show("有错误"); }
               [font color=#FF0000] serialPort1.Open();
[/font]                int a1 = 34;
                int b1 = 91;
                int c1 = a1 + b1;

                byte[] t = new byte[5];
                t[0] = 0x10;
                t[1] = 0x5B;
                t[2] = Convert.ToByte(a1.ToString("X"), 16);
                t[3] = Convert.ToByte(c1.ToString("X"), 16);
                t[4] = 0x16;
                serialPort1.Write(t, 0, 5);

                ////////开启接收数据线程
                ReceiveData(serialPort1);
            }

        }
    }
}