Question

I’ve got a question about accidentally hiding abstract methods.

I’m creating a basic Entity class as an interface from which to create all other entities in the game I’m working on.

From this Entity class, I have created several derived classes. There are things like MovingEntity, Trigger, Door, etc… Many of these children classes also have children derived from them. For example, MovingEntity has classes like Projectile and EnemyUnit as children.

In my base Entity class, I have methods like Update() and Render() that are abstract, because I want every entity to implement these methods.

Once I get down to the second level, however, -that’s- where I hit my question/problem. I’ll use the Trigger class, for example. Trigger derives from the base Entity class, but Trigger still has its own children (like TriggerRespawning and TriggerLimitedLifetime). I don’t want to instantiate a Trigger object, so I can keep that class abstract – I will only create objects from Trigger’s children classes. But what do I do with the abstract methods that Trigger is supposed to implement from Entity?

I thought I could just basically use the same code in Trigger as I did in Entity. Declare the same method, same name, same parameters, and just call it abstract. Then, Trigger’s children would be forced to implement the actual functions.

This didn’t work, however, because in the Trigger class, my build errors say that I am hiding the abstract methods from the base Entity class.

How can I pass down the idea of forcing the eventual children to implement these abstract methods without making all of the parents in-between implement them? Do I need to use virtual on the first round of children classes?

I haven’t been able to find a good answer on this so far, so I decided to break down and ask. Thanks in advance, guys.

Answer 1

Just don’t redeclare the methods at all – the eventual concrete classes will have to implement all the abstract methods still unimplemented all the way up the tree:

public abstract class Foo
{
    public abstract int M();
}

public abstract class Bar : Foo
{
    // Concrete methods can call M() in here
}

public class Baz : Bar
{
    public override int M() { return 0; }
}