private void DataReceivedHandler(object sender, SerialDataReceivedEventArgs e)
        {
            if (logging)
            {
                if (richTextBoxLog.InvokeRequired)
                {
                    richTextBoxLog.Invoke(new SerialDataReceivedEventHandler(DataReceivedHandler), sender, e);
                }
                else
                {
                    SerialPort sp = (SerialPort)sender;
                    string indata = sp.ReadExisting();
                    richTextBoxLog.AppendText(indata);
                    if (File.Exists(file_path))
                    {
                        using (StreamWriter sw = File.AppendText(file_path))
                        {
                            sw.WriteLine(indata);
                        }
                    }
                }
            }
        }

if (indata.Length > max_data_lenght)
    using (StreamWriter sw = File.AppendText(file_path))
                        {
                            sw.WriteLine(indata);
                        }