Choosing the right exception types, Kyiv 2010

1_meaningless_exception.cs

public class UserManager {
  public void AuthenticatedUser(string userName, string password) {
    if (userName != _userName) {
      //WTF does 'ApplicationException' mean?  It means "something didn't work"
      //That's not very helpful
      throw new ApplicationException("Invalid username");
    }
  }
}

2_built_in_exception.cs

public class UserManager {
  public void AuthenticatedUser(string userName, string password) {
    if (userName != _userName) {
      //This is better, because you're communicating "the userName parameter has something wrong with it.
      //Sometimes this is enough.
      throw new ArgumentException("userName", "Invalid username");
    }
  }
}

3_custom_exception.cs

public class UserManager {
  public void AuthenticatedUser(string userName, string password) {
    if (userName != _userName) {
      // This is the most specific.  If you need to detect and handle this specific case,
      // you need a custom exception.  Most of the time, this won't be necessary, but you
      // have to use your judgement
      throw new InvalidUsernameException(userName);
    }
  }
}