using a COM port - Close after each use, or leave always open?

Question

Till now I opened when I needed to send data, and closed right away. I get random "Access to Port" errors (although I always close the port after I use it), so I was thinking maybe to leave it always open.

What is the right approach of use, assuming that every minute or two I need to send data in some COM ports?

Thanks..

Answer 1

Calling SerialPort.Close() frequently is a mistake. Having another app steal the port away from you isn't exactly very desirable. But more problematic, and the problem you are having, is that Close() doesn't wait for a worker thread that is started by SerialPort to exit. That worker thread raises the DataReceived, PinChanged and ErrorReceived events. It takes "a while" for it to exit, could be between milliseconds and seconds. Calling Open() again will fail until that's done.

It's a flaw in the class, but induced by the common usage for serial ports. Apps don't normally close them until the app terminates. Including never, avoiding a common deadlock scenario. Do note that the MSDN article for Close warns about this:

The best practice for any application is to wait for some amount of time after calling the Close method before attempting to call the Open method, as the port may not be closed instantly.