How to implement an interface to a List object

Question

Ok, the question is really general. I have a generic interface:

public interface IGizmo<T>

and a class implementing the interface

public class A:IGizmo<A>

How can I implement the interface

IGizmo<List<A>>

on objects of the form:

List<A> L

Or if the above is not possible, is there a "natural way" to implement an interface to a list object class from an interface defined on the class?

Since `List` is not under your control, this isn't possible. The question is probably too generic (pardon the expression) -- the construction `A : IGizmo` is an unusual one with typically very specific use cases (like `IEquatable`) and it's hard to say what the right way to "fix" things is without knowing what you're using it for. (To consider the parallel with `IEquatable`, there is no concept of an "equatable list" as such, but then there is `IStructuralEquatable` and `StructuralComparisons`, which achieves the underlying goal.) — Jeroen Mostert, Jan 29 '20 at 13:39
You can, for example, have a method taking an `IEnumerable where T : IGizmo`, requiring that not the collection but the item implements a particular interface; a `List` would then satisfy this constraint. If you really want, you can introduce a wrapper type specifically to implement `IGizmo>` for any enumerable or list type, by delegation. — Jeroen Mostert, Jan 29 '20 at 13:43
Do you have a specific use case in mind, or are you just asking out of curiosity? For the former, it might be an [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). For the latter, the answer would be: no. You cannot change `List`, so you cannot let it implement new interfaces. Also, why (only?) `List`? What about arrays (`T[]`)? Or anything else that implements the `IList` interface? Or `ICollection`? Or even `IEnumerable`? `List` is just _one_ implementation of "a bunch of T" with a very specific set of rules. — Corak, Jan 29 '20 at 14:38
@Corak yes curiosity and also trying to understand how the language works (what are these objects intensions, what actually an interface is etc) — Squaresharper, Jan 29 '20 at 14:58
Since `List` in particular is available for subclassing, you can have `class GizmoList : List, IGizmo> where T : IGizmo {} `. Whether that's meaningful in your scenario is another matter, though, and this works for `List` specifically because you can inherit from it (which is not the case for all collection types). — Jeroen Mostert, Jan 29 '20 at 15:11
@Squaresharper - Curiosity is awesome! But sometimes an idea turns out to not be possible/feasible. As for what interfaces are, they are usually described as "contracts". For some time I misunderstood a class implementing an interface as that class saying: "look what I can do!". Now, I see it the other way round. If a method accepts an interface as parameter, the method is saying: "look what I **need**!". The method doesn't care about the actual implementation, it just needs _something_ with the right properties/methods to do its thing. And that need is communicated as an interface. — Corak, Jan 29 '20 at 15:14
@Corak -This is really true. Thank you for the tip. I think it is a very good thing to have in mind when using an interface. — Squaresharper, Jan 30 '20 at 12:10

score 0 · Answer 1 · answered Jan 29 '20 at 13:44

0

Your interface is a Generic interface, your implementation could be:

public class ObjectWithoutList : IGizmo<A>

public class ObjectWithList : IGizmo<List<A>>

Without more context I believe you've misunderstood generics? If not could you give more info?

Otherwise see these links for more info on Generics:

Pluralsight Course

Microsoft Docs

answered Jan 29 '20 at 13:44

PWT

71
3

I think your answer is in the correct direction. What I want to test is if something like the following can be made: `public class A : IGizmo` `public class List : IGizmo>` I don't know if something like this makes sense (forgive me but i am a primer in oop and c#) – Squaresharper Jan 29 '20 at 14:07
@Squaresharper - For that, you would either need to change the implementation of `List` (which you can't, as Jeroen explained), or you would need to create your own "List" class, where the question would be: why? Why would you want to do that, or are you [misunderstanding what a list is](https://stackoverflow.com/questions/21692193/why-not-inherit-from-listt)? – Corak Jan 29 '20 at 14:21

How to implement an interface to a List object

1 Answers1