unknown type name in objective c

Question

I\'m pretty new to objective c, and having some basic problems.

I wrote a simple program using a navigator, and everything worked fine. then I added few lines of cod

Answer 1

This problem looks like a typo because class names usually start with an uppercase character. Therefore, mainController could/should be MainController. Check the class name to see if the error is indeed a typo, because the compiler is telling you it cannot find any class called mainController.