public partial class Form1 : Form
    {
        System.IO.Ports.SerialPort port = new System.IO.Ports.SerialPort();  

        public Form1()
        {

            InitializeComponent();
            port.BaudRate = 9600;
            port.Parity = Parity.None;
            port.DataBits = 8;
            port.StopBits = StopBits.One;
            port.Handshake = Handshake.RequestToSend;
            port.DtrEnable = true;
            port.RtsEnable = true;
            port.NewLine = System.Environment.NewLine;

            port.PortName = "COM1";
            port.WriteTimeout = 500; port.ReadTimeout = 500;

            port.Open();// открываем порт

        }

       
        
  

    }

public Form1()
{
InitializeComponent();

Myport.BaudRate = 9600;
Myport.Parity = Parity.None;
Myport.DataBits = 8;
Myport.StopBits = StopBits.One;
Myport.Handshake = Handshake.RequestToSend;
Myport.DtrEnable = true;
Myport.RtsEnable = true;
Myport.NewLine = System.Environment.NewLine;

Myport.PortName = "COM1";
Myport.WriteTimeout = 500;
Myport.ReadTimeout = 500;

}

private void button1_Click(object sender, EventArgs e)
{
string s1;
// кнопка чтения данных с порта COM3
Myport.Open();
try
{
s1 = Myport.ReadExisting();

}
catch (Exception ex)
{
MessageBox.Show("Ошибка чтения из порта: " + ex.Message);
return;
}
MessageBox.Show(s1.ToString());
MessageBox.Show("Успешно прочитали из порта");
Myport.Close();

string[] sp = System.IO.Ports.SerialPort.GetPortNames();
for (int i = 0; i < sp.Length; i++)
{
listBox1.Items.Add(sp[i]);
}