how to give licensing for the windows application in C#.Net

When your application start at first time, at that time create a raw key which will contain
MAC address of system,
Current DateTime, and
additional details.

Then encrypt the Raw key with any algo.

Create another application (License Generator) which will decrypt the raw key and generate a license key for you.

License key will contain the details of :
MAC address(from raw key)
Current DateTime(from raw key)
Application Type (Demo or License) (from License Generator)
If demo that Expiry days (from License Generator)

Then encrypt the license details

Then take that license key and put it into actual application.

On every start you can check the expiry days.

You can store the Raw Key and License Key in Database, application Properties or in config file.

As for your problem regarding tracking the date installed for trial versions, you may refer here[^] as this was also a question asked before.

A friendly reminder, never post your questions as a solution. Rather, post it as a comment to the solutions, or you can create a new question if its a different issue.

Use a ready-to-use scheme like CryptoLicensing for this - it supports trial licenses (x days, X unique days, etc), then you can issue them full licenses without any trial limits. It comes with a ready "evaluation info dialog" which can show "X of Y days remaining," and also allow user to enter the full license code.

1. Saved Dates for license should be in encrypted form in database table.
2. License Period information should be get from database table and should be decrypted to calculate difference of days
3.A user can change system date but cant change database server dates and if a user will change database server date then user cant decrypt already encrypted dates values stored in database table.
hope it helps or let me know for description and explanation with code.