envoi byte sur port com usb

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Threading.Tasks;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;          // added to include serial ports
using System.IO;
using System.Management;

namespace RELAY_MODULE_TEST
{
    public partial class Form1 : Form
    {
        public delegate void AsyncCallback(IAsyncResult ar);
        public delegate void AddTolstDiscoveredDevices(object o);
        enum modules { NONE = 0, USB_RLY02};
        int module_id = (int)modules.NONE;
        static SerialPort USB_PORT;
        byte[] SerBuf = new byte[10];
        byte serial_written_flag = 0;
       

        public Form1()
        {
            InitializeComponent();
            USB_PORT = new SerialPort();
            string[] comportnames = new string[100];
            comportnames = SerialPort.GetPortNames();
            var sortedList = comportnames.OrderBy(port => Convert.ToInt32(port.Replace("COM", string.Empty)));

           foreach (string port in sortedList)
            {
               comboBox_comport.Items.Add(port);                      // places each "COMx" name into combobox
               comboBox_comport.Text = "COM4";
            }
            
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            switch (module_id) //module specific code
            {
                case (int)modules.USB_RLY02:
                   
                   break;
            }
            if (module_id != (int)modules.NONE)     //if any module is connected then this is the common code
            {
                SerBuf[0] = 0x5B;       // get states command
                transmit(1);
                receive(1);

                if ((SerBuf[0] & 0x01) == 0x01)
                {
                    button_relay1.Text = "RELAY ON";
                    button_relay1.BackColor = Color.Red;
                }
                else
                {
                    button_relay1.Text = "RELAY OFF";
                    button_relay1.BackColor = Color.White;
                }        
           }
        }

        private void comboBox_comport_SelectedIndexChanged(object sender, EventArgs e)  //opens a comport or network stream corresponding with combobox selection
        {
            module_id = (int)modules.NONE;
            
                if (USB_PORT.IsOpen) USB_PORT.Close();        // close any existing handle
                USB_PORT.PortName = comboBox_comport.Text;    // retrieves "COMx" from selection in combo box
                USB_PORT.Parity = 0;
                USB_PORT.BaudRate = 19200;
                USB_PORT.StopBits = StopBits.Two;
                USB_PORT.DataBits = 8;
                USB_PORT.ReadTimeout = 50;
                USB_PORT.WriteTimeout = 50;
                USB_PORT.Open();

                SerBuf[0] = 0x5A;       // get version command for RLY16, returns module id and software version
                transmit(1);
                receive(2);

                if (SerBuf[0] != 255)  // if wee recieved the module id  
                {
                    module_id = SerBuf[0];
                }
            
        }

        private void transmit(byte write_bytes)
        {
                try
                {
                    if (USB_PORT.IsOpen) USB_PORT.Write(SerBuf, 0, write_bytes);      // writes specified amount of SerBuf out on COM port
                }
                catch (Exception)
                {
                    USB_PORT.Close();
                    module_id = (int)modules.NONE;
                }
        }

        private void receive(byte read_bytes)
        {
            byte x;
                for (x = 0; x < read_bytes && USB_PORT.IsOpen; x++)       // this will call the read function for the passed number times, 
                {                                      // this way it ensures each byte has been correctly recieved while
                    try                                // still using timeouts
                    {
                        USB_PORT.Read(SerBuf, x, 1);     // retrieves 1 byte at a time and places in SerBuf at position x
                    }
                    catch (Exception)                   // timeout or other error occured, set lost comms indicator
                    {
                        USB_PORT.Close();
                        SerBuf[0] = 255;
                        MessageBox.Show("read fail");
                        module_id = (int)modules.NONE;
                    }
                }
        }

        private void button_relay1_Click(object sender, EventArgs e)
        {
                if (button_relay1.Text == "RELAY ON") SerBuf[0] = 0x6F;                   
                else SerBuf[0] = 0x65;
                transmit(1);
        }
         }
}

        private void button_relay1_Click(object sender, EventArgs e)
        {
                if (button_relay1.Text == "RELAY ON") SerBuf[0] = 0x6F;                   
                else SerBuf[0] = 0x65;
                transmit(1);
        }

        public Form1()
        {
            InitializeComponent();
            USB_PORT = new SerialPort();
            string[] comportnames = new string[100];
            comportnames = SerialPort.GetPortNames();
            var sortedList = comportnames.OrderBy(port => Convert.ToInt32(port.Replace("COM", string.Empty)));

           foreach (string port in sortedList)
            {
               comboBox_comport.Items.Add(port);                      // places each "COMx" name into combobox
               
            }
            comboBox_comport.Text = "COM4";//j'ai sortit ça de la boucle pour optimiser le code


            //ici j'appelle la méthode du bouton
            button_relay1_Click(this, new EventArgs);//la méthode demande un objet je lui est mis this qui correspond à l'instance en cours du formulaire et une variable donnant des détails sur l'événement en cours, comme je n'en ai pas j'en crée une toute neuve.
        }

Comme je l'ai écrit plus haut je n'ai pas de port com, je n'ai pas testé mais ça devrait marcher.

this.Close()