class C_Serial
{
    SerialPort Serial;

    int i_RecvSize = 0; // number of receive

    public bool OpenSerial(string s_port, int i_baud, int i_commTry, int i_delay)
    {
                  try
                  {
                                Serial = new SerialPort();
                                ....
                                Serial.DataReceived += new SerialDataReceivedEventHandler(Serial_DataReceived);
                                Serial.Open();
                  }
                  catch (Exception ex)
                  {

                  }
                  return is_ok;
    }

    void Serial_DataReceived(object sender, SerialDataReceivedEventArgs e)
    {
        i_RecvSize = Serial.BytesToRead;
    }

    private void serialWrite(string s_sendPkt)
    {
        try
        {
                      byte[] arr_sendByte = s_sendPkt.HexToByteArray();
                      Serial.Write(arr_sendByte, 0, arr_sendByte.Length);
        }
        catch (Exception ex)
        {
                      Trace.WriteLine(ex.Message + " / " + ex.StackTrace);
        }
    }

    public string SendAndRecvModem(string s_sendPkt, bool is_startPC)
    {
        s_recvPkt = waitDataReceivedModem();
    }

    private string waitDataReceivedModem()
    {
        byte[] arr_recvByte = new byte[0];
        try
        {

            while (true)
            {
                if (i_RecvSize > 0 || arr_recvByte.Length > 0)
                {
                    byte[] arr_buff = new byte[i_RecvSize]; // make array for receive
                    Serial.Read(arr_buff, 0, arr_buff.Length); // here error occurs
                    i_RecvSize = i_RecvSize - arr_buff.Length;
                    arr_recvByte = arr_recvByte.Concat(arr_buff).ToArray(); // save receive packet
                    ......
        }
        catch(Exception ex)
        {
            logToMain("Error : " + ex.Message + "/" + ex.StackTrace, COLOR.RED);
        }
        return s_recvPkt
    }// waitDataReceived
} // class

C_Serial c_Serial = new C_Serial();
 ...
 string packet = ...;
 c_Serial.SendAndRecvModem(packet);