Need to check if user exists for file permission changes

[abehm]

Hey everyone,
I am trying to figure out to do a security check to see if a user account exists in order to change read/write/execute permissions for files and folders.
Right now I just have a textbox that accepts a username, but if that user doesn't exist, when the program runs it fails because there is no mapping to user accounts.
Does anyone know how to check that a user exists?

[Plater]

I think you can do it with the ActiveDirectory stuff (LDAP?), although if it "fails", that could be your indication that the user does not exist.
But I am not sure. I also would like to here the answer(s), so I am subscribing.