C# communicating with Arduino COM access denied

Question

I'm working with a fingerprint sensor on Arduino, but sometimes, on runtime, my WPF app throws an exception on sp.Open() saying "COM3" Access denied"

Here's the code on c#:

public string ConfigPort()
    {
        SerialPort sp = new SerialPort();

        sp.BaudRate = 9600;
        sp.PortName = AutodetectArduinoPort();
        sp.Open();

        string s = "";
        while (true)
        {
            s = sp.ReadLine();
            Console.WriteLine(s);

            return s;

        }
    }

Here's the AutodetectArduinoPort method:

public string AutodetectArduinoPort()
    {
        ManagementScope connectionScope = new ManagementScope();
        SelectQuery serialQuery = new SelectQuery("SELECT * FROM Win32_SerialPort");
        ManagementObjectSearcher searcher = new ManagementObjectSearcher(connectionScope, serialQuery);

        try
        {
            foreach (ManagementObject item in searcher.Get())
            {
                string desc = item["Description"].ToString();
                string deviceId = item["DeviceID"].ToString();

                if (desc.Contains("Arduino"))
                {
                    return deviceId;
                }
            }
        }
        catch (ManagementException e)
        {
            /* Do Nothing */
        }

        return null;
    }

Any solution??

Answer 1

This happens quite a lot with Arduino, personally I think there's a problem with the FTDI driver....certainly wouldn't be the first issue with them. What I do when this happens is physically unplug the device from the USB port, wait for the notification sound from Windows and then plug it back in again. This seems to "reset" the port and force whatever is holding it open to release it, and you're good to go again. You may need to do this quite a bit over time, so the only other tip I have is to use a cheap USB hub so that you wind up wearing out the port pins on that rather than your expensive laptop/desktop.