Declare a variable auto incremental and write/read txt files

[Lusitan]

Hello.

I want to know if someone can help me with this:

I'm starting to write some code to run an office (not professionally).
When I write the Client Class, how do I make it that the number of the client be a primary key (unique and auto incremental)?

Also, I want that all the Client and Products data be stored on a txt file.

Any thoughts?

Thank you in advance.

L