Sharing a class property (field) between applications

Question

I have an 8 bit digital output board used for device controlling. Each external device needs one bit and is controlled by a different application. I have written a class library and the class DigitalOutputPort (VB 2010) that envelopes the driver that manages the 8 bit port. Each device application uses this class, creating its own instance.

In order to set a bit of the digital output port, I have to write a byte to that port: this byte is the bit mask for all 8 bits together: to set HIGH the bit number 0 - 1 - 2, I have to write 7 on the port, to set HIGH all 8 bits, I have to write 256, and so on...

All works fine when only ONE application uses the class. But if two applications want to set its own bit on that port, I get problems because I do not know the current value of all bits set by other applications (the driver has not such function) and, of course, I cannot change one bit without changing all the other (if I do not know the current bit mask)

Normally this looks like a typical case of sharing data between two application and my first idea was to write the current value of the port in a file on the disc, where all application can access and read it. But it seems to be too much heavy for this simple problem. Moreover it could also creates performance ans reliability problem.

Then I though about using a shared field (property) in the class. A shared field preserves its value between all instances of a class: but is it also true between instances from different applications? I cannot find more info about this last point, I have to make same test.

A third way would be that I create just only ONE instance of the class DigitalOutputPort, one for all applications. The first application that needs it, create the object, all other applications will used the already created object. I like more than other this way, but I do not know if and how it can be done.

Which should be the right approach in your opinion?

Thank you for replying.

Answer 1

Two different applications will always have distinct and separate memory. So even a Shared field will not be the same. A Shared field is shared only in the context of a specific application and its memory, not globally on the system.

So you need to share data between two applications. There are several options, though the simplest and easiest is the one you mentioned - store it in a file on disk. It's not overkill, since it's a very simple implementation. Just remember not to keep a lock on the file, since several processes will need to access it.

Another possibility you've raised is with a shared instance of DigitalOutputPort. This means having the first application create the instance, and expose it via WCF/Remoting/some other cross-process communication method so that other apps will access it. It's certainly possible (though the state of the DigitalOutputPort will be lost once all of these apps are closed), but it's a lot more complicated, especially if you don't already work with these communication frameworks.

I'd stick to a file on disk, or perhaps a registry key, to store shared, persistent data between applications.