无法实现接口成员，因为它没有匹配的返回类型

Question

I have some troubles with interface and inheritance. Here my problem :

I have two interfaces :

public interface IElementA
{
    List<IElementA> Child { get; }
}

// The goal is to add some properties to the main interface
public interface IElementB : IElementA
{
    string Name { get; }
}

and a class which implement IElementB

public class ElementB : IElementB
{
    protected List<ElementB> m_Child = new List<ElementB>();

    public List<ElementB> Child { get { return m_Child; } }
    public string Name { get { return "element B"; }
}

Then I got the error:

'ElementB' does not implement interface membre 'IElementA.Child'.

'ELementB.Child' cannot implement 'IElementA.Child' because it does not have the matching return type of 'List<IElementA>'."

I understand that I need to write

public List<IElementA> Child { get { return m_Child; } }

And know the template trick but it's work only with a List of different type of IElementA.

Do you have some ideas to solve my problem ?

Best Regards JM

Answer 1

You can use generics:

public interface IElementA<T>
{
    List<T> Child { get; }
}

public interface IElementB
{
    string Name { get; }
}

public class ElementB : IElementA<ElementB>, IElementB
{
    protected List<ElementB> m_Child = new List<ElementB>();

    public List<ElementB> Child { get { return m_Child; } }
    public string Name
    {
        get { return "element B"; }
    }

}

---

or if you really see inheritance here (I don't):

public interface IElementB<T> : IElementA<T> where T: IElementA<T> ...

public class ElementB : IElementB<ElementB> ...

Answer 2

If you respect the Iterface implementation your list will look to :

    protected List<IElementA> m_Child = new List<IElementA>();
    public List<IElementA> Child { get { return m_Child; } }

So you will be able to add ElementB element into it :

this.m_Child.Add(new ElementB());

If you want only ElementB in this list, check the type before insertion on it.