1)

QuickOPC is written in .NET, and the memory is managed by the CLR. You cannot expect to behave it in deterministic manner. Unless your app shows an increasing memory usage over a long period for no reason (and by long, I mean really long - 72 hours or even more), there is nothing to be concerned about.

COM applications can cause a memory leak by not releasing the objects properly. If you just subscribe/unsubscribe, however, the items are identified by handles and not COM references,, so that's not the issue here probably.

2)

Yes, you can use the component from more threads. You can do that with one instance (in which case many calls on that instance will be serialized) or with several instances (less serialization).

Regards