将一个子类分配给一个接口，涉及泛型和协变量

Question

I defined 3 interfaces:

public interface IManufacturerInput
{
}
public interface IManufacturerOutput
{
}
public interface IManufacturerApi<in S, out T>
    where S : IManufacturerInput
    where T : IManufacturerOutput
{
    T Calculate(S);
}

And I defined a specific Manufacturer:

public class ManufacturerAInput : IManufacturerInput
{
}
public class ManufacturerAOutput : IManufacturerOutput
{
}
public class ManufacturerAApi : IManufacturerApi<ManufacturerAInput, ManufacturerAOutput>
{
     public ManufacturerAOutput Calculate(ManufacturerAInput)
     {
         return null;
     }
}

And In Main() I created a ManufacturerAApi, and try assign it to IManufacturerApi.

IManufacturerApi<IManufacturerInput, IManufacturerOutput> api = new ManufacturerAApi();

But it failed. The error message said (just abstract meaning):

Can't convert from ManufacturerAApi to IManufacturerApi<IManufacturerInput, IManufacturerOutput>

So is there any way I can make the assignment work? Thanks in advance.

Answer 1

What you are proposing isn't type safe. Let's change the names of your types to make the issue clearer:

public interface IPetFood { }
public interface IPetSound { }

public interface IPetCage<in S, out T>
    where S : IPetFood
    where T : IPetSound
{
    T Feed(S s);
}

public class DogFood : IPetFood { }
public class CatFood : IPetFood { }
public class Bark : IPetSound { }

public class DogCage : IPetCage<DogFood, Bark>
{
    public Bark Feed(DogFood input)
    {
        return new Bark();
    }
}

And now suppose this is legal:

IPetCage<IPetFood, IPetSound> api = new DogCage();

Then we could do the following:

api.Feed(new CatFood()); //oops we've just given the dog some catfood.

The assignment will not work because S is contravariant, which means that any possible IPetFood passed into api.Feed would need to be a subtype of DogFood and you have the opposite; IPetFood is a superset of DogFood .