Question

Can someone please explain why I am getting a compile error asking for an explicit cast? Since I’m constraining the output, I thought this would be allowed without any brute force.

** THANK YOU EVERYONE. IT WAS CONFUSING THE WAY I PRESENTED THE QUESTION. **

public interface IQuery<TInput, TOutput>
{
    TOutput Execute(TInput input);
}

public abstract class PagedQuery<TInput, TOutput> : IQuery<TInput, TOutput>
    where TOutput : IEnumerable<TOutput>
{
    public TOutput Execute(TInput input)
    {
        return Enumerable.Empty<TOutput>(); // error here..
    }
}

Answer 1

Enumerable.Empty<TOutput>() returns an implementation of IEnumerable<TOutput>. Just because TOutput also implements IEnumerable<TOutput> doesn’t mean that you can convert any IEnumerable<TOutput> value to TOutput.

I agree with the commentators who say that constraining TOutput to return a sequence of itself is pretty odd, by the way.

I suspect you actually want something like this:

public abstract class PagedQuery<TInput, TElement>
    : IQuery<TInput, IEnumerable<TElement>>
{
    public IEnumerable<TElement> Execute(TInput input)
    {
        return Enumerable.Empty<TElement>();
    }
}

That makes a lot more sense to me.

EDIT: I’ve renamed the type parameter to make it clearer. So for IQuery<,>, TOutput=IEnumerable<TElement> – so a PagedQuery returns a sequence of elements, not a sequence of pages, each of which itself is a sequence of pages, each of which itself is a sequence of pages ad infinitum.