public partial class Form1 : Form
    {
        private Conn conn = new Conn();

        public Form1()
        {
            InitializeComponent();
        }

        private void ReadData()
        {
            while (true)
            {
                string msg = "";
                if (conn.Receive(ref msg, 200))
                {
                    if (0 < msg.Length)
                    {
                        textBoxMsg.AppendText(msg + "\r\n");
                        msg = "";
                    }
                }
                Thread.Sleep(100);
            }
        }

        private void button_open_Click(object sender, EventArgs e)
        {
            conn.Init(1,115200,"8021");
            if (conn.Connect())
            {
                textBoxMsg.AppendText("连接成功\r\n");
            }
            else
            {
                textBoxMsg.AppendText("连接失败\r\n");
            }
        }

        private void button_close_Click(object sender, EventArgs e)
        {
            conn.Close();
            textBoxMsg.AppendText("连接关闭\r\n");
        }

        private void button_auto_read_Click(object sender, EventArgs e)
        {
            Thread readThread = new Thread(new ThreadStart(ReadData));
            readThread.IsBackground = true;
            readThread.Start();
        }

        private void button_isConn_Click(object sender, EventArgs e)
        {
            if (conn.Connected())
            {
                MessageBox.Show("已连接上");
            }
            else
            {
                MessageBox.Show("正在连接...");
            }
        }

        private void button_send_Click(object sender, EventArgs e)
        {
            if (conn.Send(textBoxSend.Text))
            {
                MessageBox.Show("发送成功");
            }
            else
            {
                MessageBox.Show("发送失败");
            }
        }
    }
 

dll

public class Conn
   {
       private int port;
       private int baudRate;
       private bool isConnected = false;
       private string tel;
       private Thread stat = null;

       #region C# com 串口通信

       private mycom com = new mycom();
       /// <summary>
       /// 连接状态
       /// </summary>
       private void ConnStat()
       {
           while(true)
           {
               if (-1 < Encoding.ASCII.GetString(ComRead(200)).IndexOf("CONNECT"))
               {
                   isConnected = true;
                   stat.Abort();
               }
               Thread.Sleep(50);
           }
       }
       /// <summary>
       /// 打开串口
       /// </summary>
       /// <returns></returns>
       private bool OpenCom()
       {
           bool flag = false;
           com.PortNum = port;
           com.BaudRate = baudRate;
           com.ByteSize = 8;
           com.Parity = 0;
           com.StopBits = 1;
           com.ReadTimeout = 5000;
           try
           {
               if (com.Opened)
               {
                   com.Close();
                   com.Open();
               }
               else
               {
                   com.Open();
               }
               flag = true;
           }
           catch
           {
           }
           return flag;
       }

       /// <summary>
       /// 关闭串口
       /// </summary>
       private void CloseCom()
       {
           com.Close();
       }

       /// <summary>
       /// 向COM发送数据
       /// </summary>
       /// <param name="data"></param>
       private void ComSend(byte[] data)
       {
           com.Write(data);
       }

       /// <summary>
       /// 从COM读取数据
       /// </summary>
       /// <returns></returns>
       private byte[] ComRead(int len)
       {
           return com.Read(len);
       }
    #endregion

    /** 初始化通讯 */
     public void Init(int port, int baudRate,string tel)
    {
        this.port = port;
        this.baudRate = baudRate;
        this.tel = tel;
    }
 
    /** 清理通讯 */
    public void Clean()
    {

    }
 
    /** 连接服务器 */
    public bool Connect()
    {
        bool flag = false;
        if (OpenCom())
        {
            Send("ATDT" + tel + "\r\n");
            Thread.Sleep(20);
            stat = new Thread(new ThreadStart(ConnStat));
            stat.IsBackground = true;
            stat.Start();
            flag = true;
        }
        return flag;
    }
 
    /** 检测是否已经连接 */
    public bool Connected()
    {
        return isConnected;
    }
   
    /** 断开连接 */
    public void Close()
    {
        CloseCom();
    }
 
    /** 发送数据 */
    public bool Send(string buffer)
    {
        bool flag = false;
        try
        {
            ComSend(Encoding.ASCII.GetBytes(buffer+"\r\n"));
            flag = true;
        }
        catch
        {
            throw (new ApplicationException("send data error"));
        }
        return flag;
    }
  
    /** 接收数据 */
    public bool Receive(ref string buffer, int len)
    {
        bool flag = false;
        try
        {
            buffer = Encoding.ASCII.GetString(ComRead(len));
            flag = true;
        }
        catch
        {
            throw (new ApplicationException("read data error"));
        }
        return flag;
    }
   }
}
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