i have a page called rateuser.aspx. i would like to set a validation such that user A cannot give feedback & rating to himself. how do i do that? because user A can choose the users from a dropdownlist. so how do i validate such that when user A press themselves, an error would pop and say "you cannot choose yourself."

If you know who the user is when you populate the dropdownlist, why not keep "himself" from being on that list. 

how do i do that?

yoghurtculture

how do i do that?

That depends upon how you are populating the dropdown in the first place.  Could you please post all your related markup and code?

(It makes things a lot more readable if you use the Insert Code tool. If you need instructions, see my How to use the Forums' "Insert Code" tool. post.)