How to make licence file for one year in c# .net?

Easiest way;
1) Find yourself 16 length password string which is hardcoded in ur app.

2) Use Rjndael of .net to create ur encrypt decrypt functions.

3) Create a file with any extension and write expiration date string(which is encrypted first with ur password) into it.

4) Program loads this file, if decryption succeeds it creates a copy of file under appdata folder.

5) Each time app starts, first check if this file exists; if exists decrypt it, convert to datetime and compare with datetime.now, if expired license over else show full version.
And don't forget to obfustucate your assembly.

Notes:
And keep in mind, this license file is not machine specific and works for all. if you want to create license file specific to machine, you need to create unique license files per machine with using fingerprint of it.