Using abstract class as a generic type

Question

Is there any way to use an abstract class as a generic type? I've given a very simple example below to simulate what I need. I get an error that an implicit conversion cannot be made for variable genericBehavior.

public abstract class AnimalBase { } // Cannot edit
public class Dog : AnimalBase { }   // Cannot edit
public class Cat : AnimalBase { }   // Cannot edit

public interface IAnimalBehavior<T> where T : AnimalBase { }
public class CatBehavior : IAnimalBehavior<Cat> { }
public class DogBehavior : IAnimalBehavior<Dog> { }

public class Startup
{
    public Startup()
    {
        IAnimalBehavior<Cat> catBehavior = new CatBehavior();

        IAnimalBehavior<AnimalBase> genericBehavior = new CatBehavior(); // This doesn't work
    }
}

Ultimately, in my .NET Core Startup, I'd like to be able to have the following:

services.AddScoped<IAnimalBehavior<AnimalBase>, CatBehavior>();

or

services.AddScoped<IAnimalBehavior, CatBehavior>();

What's the best approach?

@Xerillio In fact, as it is written now, `IAnimalBehavior` is invariant, not covariant. Making it covariant would solve the problem: `public interface IAnimalBehavior where T : AnimalBase { }` — Klaus Gütter, Aug 06 '22 at 12:22
Does this answer your question? [Understanding Covariant and Contravariant interfaces in C#](https://stackoverflow.com/questions/2719954/understanding-covariant-and-contravariant-interfaces-in-c-sharp) — Klaus Gütter, Aug 06 '22 at 12:26

score 1 · Answer 1 · answered Aug 06 '22 at 12:22

Since are using IAnimalBehavior<AnimalBase>, and not IAnimalBehavior<Cat>, then this structure works:

public interface IAnimalBehavior{ }
public class CatBehavior : IAnimalBehavior { }
public class DogBehavior : IAnimalBehavior { }
...

services.AddScoped<IAnimalBehavior, CatBehavior>();

Having IAnimalBehavior<T> makes sense if you differentiate on T:

public class CatBehavior : IAnimalBehavior<Cat> { }
public class DogBehavior : IAnimalBehavior<Dog> { }

...

services.AddScoped<IAnimalBehavior<Cat>, CatBehavior>();
services.AddScoped<IAnimalBehavior<Dog>, CatBehavior>();

Using abstract class as a generic type

1 Answers1