Question

I want to have an interface specify that any implementations of that interface must use a subtype of a specific interface in their method declaration:

interface IModel {} // The original type

interface IMapper {
    void Create(IModel model); // The interface method
}

So now I want my implementation of this interface to expect not IModel itself, but a subtype of IModel:

public class Customer : IModel {} // My subtype

public class CustomerMapper : IMapper {
    public void Create(Customer customer) {} // Implementation using the subtype
}

At the moment I’m getting the following error:

‘CustomerMapper’ does not implement interface member ‘IMapper.Create(IModel)’

Is there a way I can achieve this?

Answer 1

You need to make your interface generic in the type of value it should expect:

interface IMapper<T> where T : IModel
{
    void Create(T model);
}

...

public class CustomerMapper : IMapper<Customer>
{
    public void Create(Customer model) {}
}

If you don’t make it generic, anything which only knows about the interface couldn’t know what kind of model would be valid.