You could use some of the ideas here:
http://www.craigberntson.com/Articles/kb003.htm>I am developing an application which will work with license, so that if my customers bought 5 licenses, only 5 users should be able to connect to the application server concurrently.
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>I can't figure out how to handle this, is there a library to help me with this or someone has an idea
Craig Berntson
MCSD, Microsoft .Net MVP, Grape City Community Influencer