关闭 USB 串行端口会使端口不可用

Question

My app uses USB based serial ports to connect to physical hardware devices. I can open any valid USB port and communicate with the external devices. However, when I close the connection, the USB port is left in some sort of indeterminate state for some time, and during that time further attempts to reconnect result in the "Access to port "COM--" is denied" error. However, after some few seconds, attempting to reconnect is successful. How can I determine WHEN the USB port will again support a new connection?

The code looks like this:

    private void Setup(string Port)
    {
        bool ValidPort = false;
        int CloseSleep = 10;

        _PortName = Port;
        _PortType = this;

        string[] AvailablePorts = SerialPort.GetPortNames();  

        foreach(string aPort in AvailablePorts)
        {
            if (aPort == _PortName)
            {
                // The required port is listed in the list of available ports)
                ValidPort = true;
                break;
            }
        }

        if (ValidPort)
        {
            try
            {
                if (_ThePort != null)
                {
                    _ThePort.Close();
                    _ThePort.DataReceived -= ReceivedDataEventHandler;

                    while(CloseSleep-- > 0)
                        System.Threading.Thread.Sleep(100);

                    _ThePort.Dispose();
                    _ThePort = null;
                }
            }
            catch (Exception ex)
            {
                EMS_Config_Tool.ModalDialog md = new EMS_Config_Tool.ModalDialog("Closing Port: " + ex.Message, "System Exception");
                md.ShowDialog();
            }

            System.IO.Ports.SerialPort TheNewPort = new System.IO.Ports.SerialPort(Port, 38400);

            // Setup the event handlers from Tx and Rx
            Handler.DataOutEvent    += CommsSender;
            TheNewPort.DataReceived += ReceivedDataEventHandler;

            TheNewPort.DataBits  = 8;
            TheNewPort.Parity    = Parity.None;
            TheNewPort.Handshake = System.IO.Ports.Handshake.None;
            TheNewPort.StopBits  = System.IO.Ports.StopBits.One;

            // We will try 3 times to open the port, and report an error if we fail to open the port
            try
            {
                TheNewPort.Open();
            }
            catch (Exception)
            {
                System.Threading.Thread.Sleep(1000);

                try
                {
                    TheNewPort.Open();
                }
                catch (Exception)
                {
                    System.Threading.Thread.Sleep(1000);

                    try
                    {
                        TheNewPort.Open();
                    }
                    catch (Exception ex)
                    {
                        EMS_Config_Tool.ModalDialog md = new EMS_Config_Tool.ModalDialog("Opening Port: " + ex.Message, "System Exception");

                        return;
                    }
                }
            }

The final catch statement is where the error about Access being denied is issued. Note my attempt to retry opening the port 3 times doesn't really help. If I leave the port alone for about 5 to 10 seconds and retry calling the Setup method it succeeds immediately.

Answer 1

As @Neil said, there are many issues. The best thing to do, in my point of view, is to put the search in a loop, and as soon as the port can be opened, it will be.

I used to do like this :

public Task WaitingPort()
{
    while (port is null)
        {
            port = CheckPort();
        }
}

private SerialPort CheckPort()
{
    string[] listPort = SerialPort.GetPortNames();
    foreach(string namePort in listPort)
    {
        SerialPort port = new SerialPort(namePort, 9600);
        if (!port.IsOpen)
        {
            try
            {
                port.Open();
                port.ReadTimeout = 1500;
                string data = port.Readline();
                // I programmed my device to send an "A" until it receives
                // "777" to be able to recognize it once opened
                if (data.Substring(0, 1) == "A") 
                {
                    port.ReadTimeout = 200;
                    port.Write("777"); // to make it stop sending "A"
                    return port;
                }
                else
                {
                port.Close();
                }
            }
            catch (Exception e1)
            {
                port.Close();
            }
        }
    }
    return null;
}

Of course, this is just some kind of a template which you have to reshape to your use

Answer 2

I have amended my code to use a constrained loop to give it a better chance to work, which it usually does. I was hoping that there was a better way to do it, as I tend to have pretty impatient users who will be posting defect reports if they have to wait 5 or 10 seconds to make a connection....

            // We will try several times to open the port, upto 10 times over 5 seconds, and report an error if we finally fail to open the port
            try
            {
                TheNewPort.Open();
            }
            catch (Exception ex)
            {
                RetryOpenTimer.Interval = 500;
                RetryCount = 10;
                RetryOpenTimer.Elapsed += new System.Timers.ElapsedEventHandler(RetryOpenTimer_Elapsed);
                WaitForOpen = true;
                RetryOpenTimer.Start();

                while (WaitForOpen && RetryCount > 0)
                {
                    System.Threading.Thread.Sleep(500);
                }
                if (WaitForOpen)
                {
                    EMS_Config_Tool.ModalDialog md = new EMS_Config_Tool.ModalDialog("Opening Port: " + ex.Message, "System Exception");
                    return;
                }
            }

...

    void RetryOpenTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
    {
        RetryOpenTimer.Stop();
        RetryOpenTimer.Elapsed -= RetryOpenTimer_Elapsed;

        try
        {
            if (RetryCount-- > 0)
            {
                TheNewPort.Open();

                WaitForOpen = false;
            }
            else
                return;
        }
        catch (Exception)
        {
            RetryOpenTimer.Start();
            RetryOpenTimer.Elapsed += RetryOpenTimer_Elapsed;
        }
    }