如何为C＃应用程序添加有限的时间和功能？

Question

I have desktop C# application, What is the best way to add these functionalities:

1. Limited time after which the app needs reinstall?
2. Limited privileges? (Each user will have different access to some functions of the software)

I want these options without login or server accessibility. I want something like library or dll.

Answer 1

1. It can be easily done and it is very hard at the same time.
   Why can it be easily done? You can simply write the time of the first launch \\ installation in a text file / registry / somewhere else, and check it on every launch.
   However, I would not rely on this approach since any experienced user can "reset" this limitation by deleting / modifying your file / registry key.
   If your application is commercial, then it means (often, huge) loss of money.
   Recall how different commercial applications provide trial functionality (MS Windows, MS Office, Adobe Photoshop etc.) and recall how easily they are all cracked with a 300 KB application :)

   If you want to make it safe, then you need to use generated and expirable license keys - better with license server , since offline keys generation / validation algorithm can be easily hacked as well.

2. You cannot implement a role-based system without login / computer / any other identification, because you need to differentiate your users. The only way to do this is to implement different applications for different roles.

   Priviliges / roles etc. without a server also means hard-coded priviliges, and that's very bad.
   You can create something like a resource file with information about who is administrator and who is a simple user (for a two-role system, for instance).

   Then, in your code, implement logics based on their roles:

    if (Session.User.IsAdministrator) // all are own-written classes, of course buttonRemoveUser.Show(); 

   It can be done in a way much better approach using role providers etc., it is all up to you.

   However, these users and roles can be easily cracked through a decompiler / resource viewer. Also, if any user credentials become compromised, you cannot block this user or change its password.
   The proper solution is to use server authentication .

You want to create a system with functionality, which requires client-server architecture.
Think twice before trying to implement it as offline application.
If it was possible and secure, then why would people use servers at all?