Question

I’ve written a class that implements IEnumerable<T>. I have a method that returns MyClass. If I try to yield return from within that method, the compiler tells me “… cannot be an iterator block because … is not an iterator interface type”.

So, how can I define my own interface iterator type? Does it have to be “abstract” (can’t have any methods defined)?

What I want to do is write a bunch of chainable methods, so every method should return an instance of MyClass. But I need MyClass to be some kind of enumerable. Rather than using some underlying data type, I was hoping I could just yield return everywhere.

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@Oded:

class SharpQuery : IEnumerable<HtmlNode>
{
    public SharpQuery Find(string selector)
    {
        foreach (var n in this)
        {
            // filter the results
            yield return node;
        }
    }
}

Answer 1

No, that’s not possible. To see why consider that you have a class Zoo that implements IEnumerable<Animal> but also has lots of other members. A Zoois an IEnumerable<Animal> but not necessarily vice versa – a sequence of animals is just a sequence of animals. There’s no zoo keeper, no shops, no entrance fee or any of the other things that makes a zoo a zoo.

When you use yield return x the return type cannot be Zoo because you don’t have a zoo – you just have a sequence of animals.

What you can do instead is to call it as new Zoo(foo()) where foo returns an IEnumerable<Animal> and add a constructor to Zoo that accepts an IEnumerable<Animal>.