Secure Code to prevent Re-sharing

You are right, but Only if I'm giving DLL. But what if I will share the source code? how to implement license key?

Then don't open source your project, give customer with licenced version.

If I will sale my code only to one person then how can I logically restrict to prevent distribution. In third world country there is no respect for copyrigh to written licenses.

Open source will allow anyone to use that freely. You can released under a license in which the copyright holder grants users the rights to study, change, and distribute the software to anyone and for any purpose. Open-source software may be developed in a collaborative public manner and prominent example of open collaboration.