You really can't.  Web Services provide a way to send a message from
one applciation domain to another.  All that is sent it a message, now
would you could do is have your class available on both the Server and
client side, and define your own web proxy on the client side that
would take the message cast it into your class.  This is hard to do.  I
suppose the first question is why would you want to do this?  If you
really need this maybe you should use .Net remoting