Question

I am writing a multithreaded application where a thread locks the serialport and continuously sends commands and gets responses using SerialPort.Write(command) and SerialPort.ReadLine()
in a loop untill it gets an “OK” response for one particular command.

Is it possible for the thread to switch during a ReadLine() call and cause a TimeOutException.

I have a timeout set at 2000ms.

I have random TmeOutExceptions after the program runs for a few minutes (about 5 – 10)

23332   10:14:23 AM AutoLoader.WinS IRP_MJ_WRITE    Serial0 SUCCESS Length 9: STG|4.5.. 
23336   10:14:23 AM AutoLoader.WinS IRP_MJ_READ Serial0 SUCCESS Length 1: O 
23339   10:15:54 AM AutoLoader.WinS IRP_MJ_WRITE    Serial0 SUCCESS Length 7: GTSNS..   
23342   10:15:54 AM AutoLoader.WinS IRP_MJ_READ Serial0 SUCCESS Length 3: K..   
23347   10:15:54 AM AutoLoader.WinS IRP_MJ_WRITE    Serial0 SUCCESS Length 7: GLSTS..   
23351   10:15:54 AM AutoLoader.WinS IRP_MJ_READ Serial0 SUCCESS Length 1: O 
23355   10:15:54 AM AutoLoader.WinS IRP_MJ_READ Serial0 SUCCESS Length 3: K..

After 10:14:23 AM I get a TimeOutException which I suppressed.

                    while (response != "OK")
                    {
                        Thread.Sleep(1000);

                        _alComm.SendString("SSD|" + CurrentData.MeasuredDepthReturns + Environment.NewLine);
                        response = _alComm.ReceiveString(2000);

                        _alComm.SendString("STG|" + CurrentData.AvgGas + Environment.NewLine);
                        response = _alComm.ReceiveString(2000);

                        _alComm.SendString("GTSNS\r\n");
                        response = _alComm.ReceiveString(2000);
                        SetSamplingTimePercent(response);
                    }

and

public void SendString(string sDataToSend)
{
    if (sDataToSend == "")
        return;

    try
    {
        AlPort.Write(sDataToSend);
    }
    catch (Exception ex)
    {
        //MessageBox.Show(ex.ToString(), "Error");
    }
}


public string ReceiveString(int timeOut)
{
    string inString = null;
    try
    {
        // set read timeout
        AlPort.ReadTimeout = timeOut;
        inString = AlPort.ReadLine();
    }
    catch (TimeoutException ex)
    {
        return string.Empty;
    }

    return inString;
}

Answer 1

A ReadLine (or any I/O for that matter) will cause a context-switch until it is serviced. Now, the way I see there are two possibilities:

1. Either the thread stays suspended by the OS until the timeout elapses and the exception gets thrown or
2. You stay suspended until the I/O is serviced at which point with high probability the OS will wake your thread immediately and preempt other threads to allow your code to run. Remember that any modern OS gives a generous priority boost to I/O bound threads.