How to create custom exception in objective c?

I am trying to achieve something like this in objective c.

@try{
//some code that will raise exception
}
@catch(CustomException e){//How to create this
//catching mechanism
}
@catch(NSException e){
//Generic catch
}

I need to create CustomException class and use it. Could you please help me in creating this CustomException and guide me in how to use this.

Thanks in advance.