Design By Contract uses preconditions and postconditions of the public  methods in a class together to form a contract between the class and  its clients.a) In code we implement preconditions and postconditions either as assertions or as exceptions? b) We implement preconditions and postconditions in code as exceptions if not fulfilling preconditions or postconditions doesn't indicate logically impossible situations or programming errors?c) we implement them in code as assertions when not fulfilling preconditions or postconditions does indicate logically impossible situations or programming errors?d) Should preconditions and postconditions only be defined on public methods?EDIT:Aren't the following checks considered to be part of a normal operation ( and as I already mentioned, I've seen plenty of articles on DbC using similar examples for preconditions, where checks were made against arguments supplied by the user ), since if the user enters bad data, then without checks operation won't be rejected and as such system will stop working according to specs:Link:public User GetUserWithIdOf(int id,        UserRepository userRepository) {  // Pre-conditions  if (userRepository == null)      throw new ArgumentNullException(          userRepository);  if (id <= 0)      throw new ArgumentOutOfRangeException(          id must be > 0);  User foundUser = userRepository.GetById(id);  // Post-conditions  if (foundUser == null)      throw new KeyNotFoundException(No user with  +          an ID of  + id.ToString() +           could be located.);  return foundUser;}
Assertions vs Exceptions - is my understanding of the differences between the two correct?
exceptions;assertions