Are empty abstract classes a bad practice, and why?

Question

We have several empty abstract class in our codebase. I find that ugly. But besides this very stupid reason (ugliness), should I refactor it (into empty interface e.g.) ?

Answer 1

According to object oriented programming theory the main purpose for inheritance is polymorphism, code reuse and encapsulation. An empty abstract class (and when i say this i mean truly empty, no constructors, no methods, no properties. nothing!) does not achieve any of the three goals hoped by this programming technique. It is the equivalent to if(true){...}. changing it to an interface does not really makes it any better.

If you want to refactor the code i would advise you to think in the direction opposite to the one you are thinking, what i mean by this is: try to abstract properties, methods and constructors from all classes that share a abstract parent class.

This is hard work with little reward in the short term but it increases the maintainability of the code dramatically since a core logic change would have to be done only once. I suspect the reason for using those empty abstract classes is to identify a group of classes that must share something in common otherwise what would be the difference between Object and the abstract class