c# 串口接收数据问题

namespace 串口
{
    public partial class Form1 : Form
    {
        public string ReceiveString = string.Empty;
        public string SendString = string.Empty;
        private SerialPort port = new SerialPort();
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            port.PortName = "COM1";
            port.BaudRate = 9600;
            port.Parity = Parity.None;
            port.StopBits = StopBits.One;
            port.DataReceived += new SerialDataReceivedEventHandler(SerialPort_DataReceived);
        }   
        private void btnSend_Click(object sender, EventArgs e)
        {
            try
            {
                port.Open();
                SendString = txtSend.Text;
                port.WriteLine(SendString);
                txtReceive.Text = "数据发送成功！";
            }
            catch (Exception ex)
            {
                txtReceive.Text = ex.Message;
            }
            finally
            {
                port.Close();
            }
        }
        private void SerialPort_DataReceived(object sender,SerialDataReceivedEventArgs e)
        {            
            ReceiveString = port.ReadLine();
        }

        private void btnReceive_Click(object sender, EventArgs e)
        {           
            txtReceive.Text = ReceiveString;         
        }
    }
}