Overriding Abstract Method C# for different child abstract classes

Question

public abstract class Problem
{
    public abstract List<Action> GetPossibleActions(State currentState);
}

Here both Action and State classes are also Abstract Classes.

In the child class of Problem, I am implementing that abstract method with using children of Action and State. Now it gives me error because I didnt use the same Abstract classes for return type and argument.

public class PizzaProblem : Problem
{
    public List<PizzaAction> GetPossibleActions(PizzaState currentState)
    {
       ......
    }

}

`Problem p = /* Some child class, who knows which one */; p.GetPossibleActions(/* Any possible state derived class, not necessarily related to the previous child */);`. Do you see the problem? — Damien_The_Unbeliever, Aug 18 '22 at 06:31
Does this answer your question? [Does C# support return type covariance?](https://stackoverflow.com/questions/5709034/does-c-sharp-support-return-type-covariance) — Charlieface, Aug 18 '22 at 10:26

knittl · Answer 1 · 2022-08-18T07:18:08.967

1

You could make your class a generic class:

public abstract class Problem<TAction, TState>
  where TAction : Action
  where TState : State
{
    public abstract IList<TAction> GetPossibleActions(TState currentState);
}

public class PizzaProblem : Problem<PizzaAction, PizzaState> { ...

Or don't care about the concrete type contained in your list: rely on polymorphism to call the correct methods.

edited Aug 18 '22 at 07:18

answered Aug 18 '22 at 06:23

knittl

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Additionally you can add general type constraints for `TAction` and `TState` to force them to implement `Action` and `State`. – Qwertyluk Aug 18 '22 at 06:48
How should I pass the Argument? `SomeMethod(Problem problem) ; ` `pprob = new PizzaProblem();` now it shows me an error when i call. `SomeMethod(pprob);` @knittl – hikariakio Aug 18 '22 at 08:24
@YeGaung right. In that case, you need to define an interface with co-/contravariant generic types. But I think that this approach might not be the best for your problem at hand. Why do you need to know the concrete types and cannot rely on polymorphism? – knittl Aug 18 '22 at 08:31
@knittl I just changed my code to a normal simple way, now. Now my code has a lot of casting. Is that normal? – hikariakio Aug 18 '22 at 08:43
@YeGaung what is a "normal, simple way"? – knittl Aug 24 '22 at 05:27

score 0 · Answer 2 · answered Aug 18 '22 at 06:21

The signature needs to stay the same way as in the abstract class. Try

public List<Action> GetPossibleActions(State currentState)
{
    // Your code
}

Also Action is an object for it self. If you didn't define the PizzaAction type which inherits from action, it won't work. Same is valid for State.

Overriding Abstract Method C# for different child abstract classes

2 Answers2