SerialPort.GetPortNames() behavior

In my C# 2008 application, I use the SerialPort.GetPortNames() function to retrieve the list of currently available ports. What I have noticed is, when ever I plug in a USB device, it's port number i s shown in the list on my Application and when I unplug it and refresh the list, the port number is no longer there.

One phase of the application involves reading/writing data from/to the device continuously. Now, my expectation is, If I unplug the device during the operation and get the current Port list using SerialPort.GetPortNames(), the Port Name will not be there and I can use that to make the decision that the device has been unplugged.

To my surprise, the Port name is still found despite having it removed

Why is the program behaving like this? The port name isn't listed when in no-commun开发者_运维问答ication mode. Does it have something to do with the device being removed when it's communicating?

You have to be talking about SerialPort.GetPortNames(), "GetPortList" doesn't make sense. The function iterates values in the registry, written there by your USB emulator device driver. You can have a look-see with Regedit.exe, navigate to HKLM\Hardware\DeviceMap\SerialComm. Unplug it, press F5, if the COM port is still there then SerialPort doesn't know any better than the port still being present.

There is no prescribed behavior as to how a serial port device driver should behave when the port suddenly vanishes. Serial ports are very primitive, they date back to an era where "bug" meant a moth gumming up the teletype. There is no hardware support at all for Plug and Play, removing a port with the power turned on is equivalent to unplugging the disk drive while Windows is swapping to the paging file.

Most device drivers return an error code, it generates an uncatchable exception that crashes your program. The subject of this feedback article. Apparently your device driver doesn't do that, which ought to be preferable over bombing your program. Encouraging btw, most USB emulator device drivers are utter junk.

The ultimate workaround is simple: put a little tag on the plug "don't disconnect while in use!" It's kinda of a problem with USB, most people look at it and go "hmm, what can I do with it?". And arrive at the only answer and unplug it. After a couple of kabooms, they'll learn to not do that anymore.

I'm assuming you mean System.IO.Ports.SerialPort.GetPortNames(), because i could not find a GetPortList() function anywhere. MSDN says: "If the registry contains stale or otherwise incorrect data then the GetPortNames method will return incorrect data", so that's probably where the problem lies. I guess Windows doesn't update the registry if the port is still being 'used', just like you can't delete a file when a program has a handle on it.

If you want to test if the device is removed, you can do so with a Window API call (http://www.pinvoke.net/default.aspx/user32/RegisterDeviceNotification.html). Hope that helps!

It is correct that GetPortNames() reads the ports from the Registry key

HKLM\Hardware\DeviceMap\SerialComm

This is automagically updated by Windows every time a port is opened or closed.

But nevertheless it has happened to me that there is a non-existent Port listed in the Registry, and also returned from GetPortNames(). When I try to open this port I get "The Port XYZ does not exist".

What is that ???

I now found out the reason: This happens always after using PortMon from www.sysinternals.com. This tool is buggy and lets the dead port hanging around in the Registry if the port is closed while it is monitored.

In this case the only remedy is to reboot the computer.

As others have mentioned, it's very driver-specific. There does not appear to be a way to check with the .Net API whether a port returned by GetPortNames() actually exists and is valid.

As for why the ports behave like this, I have found that some USB-to-serial drivers cause the application to crash when the port is unplugged suddenly.

Other drivers, often those that do not crash, will keep the port in the list until your application closes it, then it disappears. Trying to read from or write to the stale port will (usually) cause timeouts or errors. Presumably, in order not to crash the application, the driver needs to keep the stale port around while it is still open in the application.

If you plug in the port again, some drivers are even be able to reconnect it to your application, others will not recognize the port until your application closes the stale port. This reconnecting behaviour can be somewhat dangerous if the port disappeared because the device rebooted, because then it will suddenly be in a different state than your application expects without an obvious indication that it has reset. At least if you get errors from the port you know that something happened.

I've also found that if I forget to close the port, it won't disappear from the list until the garbage collector gets around to disposing the SerialPort object.