>I try to find how to control user licences for my application.
>I need to know how many user are using the application in a multiuser environment and take control over end user licences.
>Does anybody know how to implement this funcionality ?

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I don't, but made something myself.
Simple way:
Create a common table with a increasing number for each login.
You have to consider that this may have problems with system chrashes

Other way
You can have a common file (table/ini don't matter) where you can save a id of the current user.
Also save that id locally.
On shutdown remove it.
If you start the app look first if the id in the local file is there. If so use that id again.
if not create a new one
If you wish you can also check with a timer if the user removed the id to get more licences.

I hope this is clear cause my english is not so good.

Hope this helps,