Question

Here is my example interface:

public interface IJob<T, R>
    where T : IStep
    where R : IDelivery
{
    T Step { get; set; }
    R Delivery { get; set; }
}

Here is my example implementation

public class ImageJob<T, R> : IJob<T, R>
    where T : ImageStep
    where R : ImageDelivery
{
    public T Step { get; set; }
    public R Delivery { get; set; }
}

Both ImageStep and ImageDelivery implement their respective interfaces (IStep, IDelivery)

Now what I’m trying to do is interface all of my repository methods. Let’s take this method for example:

public void CreateJob(IJob<IStep, IDelivery> job);

So I create a new:

var job = new ImageJob<ImageStep, ImageDelivery>
{
    ...
}

And then I try to pass that into the repository method:

repository.CreateJob(job);

And I get an error saying:

Unable to cast ImageJob<ImageStep, ImageDeliver> to type IJob<IStep, Idelivery>

Can someone explain to me why this is throwing an error? Am I implementing and interfacing totally wrong?

Is there an elegant to solution to what I’m trying to do?

I want to abstract out each Job with different Step and Delivery methods, etc. Is this possible some other way? Or am I missing something entirely?

Answer 1

I think could use Covariance. This enables you to cast a specific Generic type to one of its Generic interfaces or base classes like:

MyType<object> list = new MyType<MyType>;

Class and interface decleration:

public interface IMyType<out T>{...}
public class MyType<T> : IMyType<T> {...}

The “out” keyword in the Generic decleration enables the cast.

Here is a nice page on the MSDN which may help you further

In your case it would be (if you don’t net the setters in the interface

public interface IJob<out T, out R>
    where T : IStep
    where R : IDelivery
{
    T Step { get; }
    R Delivery { get; }
}

This should work with your implemented class