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Asked: 2026-05-10T14:43:34+00:00

I’ve got a collection that implements an interface that extends both IList<T> and List.

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I’ve got a collection that implements an interface that extends both IList<T> and List. 

public Interface IMySpecialCollection : IList<MyObject>, IList { ... }

That means I have two versions of the indexer.

I wish the generic implementation to be used, so I implement that one normally:

public MyObject this[int index] { .... }

I only need the IList version for serialization, so I implement it explicitly, to keep it hidden:

object IList.this[int index] { ... }

However, in my unit tests, the following

MyObject foo = target[0];

results in a compiler error

The call is ambiguous between the following methods or properties

I’m a bit surprised at this; I believe I’ve done it before and it works fine. What am I missing here? How can I get IList<T> and IList to coexist within the same interface?

Edit IList<T> does not implement IList, and I must implement IList for serialization. I’m not interested in workarounds, I want to know what I’m missing.

Edit again: I’ve had to drop IList from the interface and move it on my class. I don’t want to do this, as classes that implement the interface are eventually going to be serialized to Xaml, which requires collections to implement IDictionary or IList…

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1 Answer

  1. You can’t do this with

    public interface IMySpecialCollection : IList<MyObject>, IList { ... }

    But you can do what you want with a class, you will need to make the implementations for one of the interfaces explicit. In my example I made IList explicit.

    public class MySpecialCollection : IList<MyObject>, IList { ... }

    IList<object> myspecialcollection = new MySpecialCollection(); IList list = (IList)myspecialcollection;

    Have you considered having IMySpecialCollection implement ISerializable for serialization? Supporting multiple collection types seems a bit wrong to me. You may also want to look at casting yout IList to IEnumerable for serialization since IList just wraps IEnumerable and ICollection.

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