Type or namespace could not found

I have problem with my c# applications i am unable to do my c# applications because of one exception that it is showig for all applications. when ever i use a data set or a parameter or sqldbtype it says that the " type or namespace name could not be found (are u missing a using directice or an assembly reference)" Please tell me how could i remove this exception which is trubling me to continue with work. I could be thank full if any body could help me to cotinue mu work.