SerialPort throwing uncatchable error?

[Chinners]

Hi, I am having a bit of difficulty trying to catch an error thrown by io.ports I have an app that displays a message on an external screen, using serial comms. The routine to do this needs to be as "bullet-proof" as possible. Testing the code, it is fine on a standard serial port, I can unplug the port and plug it back in again, and the code recovers, but as an ultimate "real world" test, I tried it on a USB->Serial adapter, and it works well - untill I unplug the USB cable and plug it back in again. Now, what happens is a bit strange. The application recovers (so the com port is coming back), and starts displaying messages again, but only for a few seconds. I then get the following exception:

System.UnauthorizedAccessException was unhandled
Message="Access to the port is denied."
Source="System"
StackTrace:
at System.IO.Ports.InternalResources.WinIOError(Int32 errorCode, String str)
at System.IO.Ports.SerialStream.Dispose(Boolean disposing)
at System.IO.Ports.SerialStream.Finalize()

I cannot trap this error, and the application just bombs out. I certainly am not disposing of the serialport object I have created, so this must be being done automatically. But, my question is, how do I catch, and trap this error? Any help, or pointers would be much appreciated. Thanks, Jason

[Dave Kreskowiak]

If you disconnect the cable, the serial port is still a device that shows up in DeviceManager. When you remove the USB device, the entire COM port goes away. The serial device no longer exists. So, when it comes back, your (internally managed) handle to the COM port is no longer valid. It may work briefly, but you've found out what happens.

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Dave Kreskowiak Microsoft MVP Visual Developer - Visual Basic
     2006, 2007, 2008

[Rajesh Anuhya]

hi jason Sorry for my poor english, I understood your problem. But here i am not telling you to do this. In this situation there is no problem with on board PCI comports. This problem happens only for the USB2Serial cables which are Run through 3rd party drivers(USB2Serial Drivers). When U connected Your USB Device to PC then only u get a comport in your Device Manager. If Your Remove this it will gone pertinently until u connect again. While Running Your Application If U Remove the USB Device, comport will removed from your PC. So, In your application before reading/writing to comport, check for comport is present or not. instead of catch the exception. Thanks

Rajesh B --> A Poor Workman Blames His Tools <--

[Chinners]

Thanks for the reply. Yes, I had thought this could be the problem, and now, when this type of error occurs, I destroy my serialport object and keep looking through the "My.Computer.Ports.SerialPortNames" collection to see if my comport reappears. If it does, I then create a new serialport object. However, I get the same problem - a crash after a few seconds. I suppose the real question I am asking is how to prevent the exception crashing my program completely? All code that accesses the comport is within a try/catch block, but the exception is still unhandled.

[Chinners]

Thanks for the reply. I have started checking that the comport is availiable however, if the cable is unplugged between checking for the port and the "open / write / close" cycle, then I cannot catch the error. The cable could be pulled out at any time. If my display stops working, that is not a problem, the fact my application stops working is VERY bad.