Is it OK for a factory method to return null?

Question

I\'m wondering about best practice here. Is it good practice for a factory method to return null if it can\'t create anything? Here\'s an example:

ICommand comma         


        

Answer 1

Returning null in this case will make your method harder to use; clients have to be aware of an implicit failure condition. Instead, throw an exception, and you can also provide a separate method for clients to test for this condition:

if (CommandFactory.CanCreate(args)) {
  ICommand command = CommandFactory.Create(args);
  command.Execute();
}

Or make the factory instantiatable; which would be better if you need to pre-process args:

CommandFactory factory = new CommandFactory(args);
if (factory.IsValid()) {
  ICommand command = factory.Create();
  command.Execute();
}

The interface of the factory now makes it clear and explicit that creation might fail, but it still requires the client to use the checking method. Another option is this:

ICommand command;
if (CommandFactory.TryCreate(args, out command)) {
  // creation succeeded ...
}