why is access to com port denied?

Question

the code:

static void Main(string[] args)
{
    Console.WriteLine("Memory mapped file reader started");

    using (var file = MemoryMappedFile.OpenExisting("AIDA64_SensorValues"))
    {
        using (var readerz = file.CreateViewAccessor(0, 0))
        {
            var bytes = new byte[567];
            var encoding = Encoding.ASCII;
            readerz.ReadArray<byte>(0, bytes, 0, bytes.Length);

            File.WriteAllText("C:\\myFile.txt", encoding.GetString(bytes));

            var readerSettings = new XmlReaderSettings { ConformanceLevel = ConformanceLevel.Fragment };
            using (var reader = XmlReader.Create("C:\\myFile.txt", readerSettings))
            {
                while (reader.Read())
                {
                    using (var fragmentReader = reader.ReadSubtree())
                    {
                        if (fragmentReader.Read())
                        {

                            reader.ReadToFollowing("value");
                            SerialPort port = new SerialPort("COM2", 9600, Parity.None, 8, StopBits.One);
                            port.Open();
                            port.Write(reader.ReadElementContentAsString() + ",");
                        }
                    }
                }
            }    
        }
    }

    Console.WriteLine("Press any key to exit ...");
    Console.ReadLine();
}

it reads shared memory, writes that shared memory to file, then the same file is opened with xml reader and splits xml since it has multiple roots, then gets the value of a node on each new split xml and sends over serial. it works on the first split xml and its node is sent over serial then it stops with a access is denied to com port message on attempt for writing second node to serial.

i have another app i made with same serial code and it works fine(i just tired it then closed it.)... so its strange.

Answer 1

You can open a serial port only once. But your code has the Open() call inside the while loop. That will only work for the first pass through the loop, kaboom on the 2nd pass. @cdhowie's solution doesn't work either, SerialPort has a quirk (aka bug) that the documentation warns about. It needs time to let a worker thread exit after the Dispose() or Close() call. The amount of time is unspecified and unpredictable.

The real solution is simple, just move the Open() call before the while loop.

Answer 2

In addition to Hans' answer:

I had the same problem and played around a bit with some sleep times between opening and closing the serial port. In my case 250 ms were sufficient. Maybe this will help out somebody out there.

EDIT:

I optimized my solution and this is what I came up with:

int maxRetries = 20;
const int sleepTimeInMs = 50;
string loggingMessage = string.Empty;

while (maxRetries > 0)
{
    try
    {
        loggingMessage = "Opening serial port '" + mSerialPort.PortName + "'...";
        mSerialPort.Open();
        loggingMessage += "Succeeded.";
        IOLogger.LogInfo(loggingMessage);
    }
    catch (UnauthorizedAccessException unauthorizedAccessException)
    {
        maxRetries--;
        loggingMessage += "Failed (UnauthorizedAccessException): ";
        IOLogger.LogError(string.Format(loggingMessage + unauthorizedAccessException.Message + " -> Retrying in about {0} milliseconds...", sleepTimeInMs));
        Thread.Sleep(sleepTimeInMs);
    }
    catch (Exception exception)
    {
        loggingMessage += "Failed: ";
        IOLogger.LogError(loggingMessage + exception.Message);
    }
}

You can play around with the sleepTimeInMs and/or the maxRetries. I have chosen those values because they seemed to be sufficient in any needed use case.