Not allow anyone to use my code

You can achieve this using assembly signing and to restrict specifically particular class, you can refer the approach mentioned here:

https://social.msdn.microsoft.com/Forums/en-US/75d6dceb-955a-4c40-a0ba-bf97632c0c91/how-can-i-use-a-dll-after-delay-signing-an-assembly?forum=csharpgeneral