>>>Is this a WinForms project ?
>>
>>Is it a Class Library.
>>
>>>The .NET framework has a built-in license manager including a (fairly simplistic) 'LicenseProvider' which you can derive from and provide your own logic.
>>>See: https://msdn.microsoft.com/en-us/library/fe8b1eh9.aspx and this article: https://www.developer.com/net/csharp/article.php/3074001/Applications-Licensing-using-the-NET-Framework.htm
>>>If the application will have internet access then your own licenseprovider could validate via a server.
>>
>>At run time, there could be various approaches. But, that project is a DLL that is included in a client project. That project comes with the source so to make sure the developers can have access and view the classes code but they will not modify it. The concept is to assure that the Class Library will only be installed as per the number of installations (PCs) the client purchased it for.
>>
>>>There are also some reasonably priced third party solutions: e.g.
>>>http://infralution.com/products/licensing_system.html
>>>http://www.ssware.com/cryptolicensing/cryptolicensing_net.htm
>>>
>>>Finally, you could consider cloud based subscription services where users pay per week/month or year. Again there are third party providers that can manage recurring payments and validate users.
>>
>>With the above given information, do you still think there is some of that that could help?
>
>If you want your classes to not be modified, then I think that's what 'SEALED' keyword in used for in C#.

On VB.NET it's 'Notinheritable'. But that only means that they can't derive from it (and thus override methods) which achieves nothing if the user has access to the source....

>Also, you should include it in the purchase agreement and if the developers will change your code, then they will violate that agreement.