Cannot implement interface member because it does not have the matching return type of List<IInterface>

Question

I have interfaces IChild and IParent. IParent has a member that is a List<IChild>.

I wish to have classes that implement IParent where each class has a member that implements IChild:

public interface IChild
{ 
}  

public interface IParent
{  
    List<IChild> a { get; set; }
} 

public class ChildA : IChild
{ 
} 

public class ChildB : IChild
{ 
} 

public class ParentA : IParent
{ 
    public List<ChildA> a { get; set; }
}

public class ParentB : IParent
{ 
    public List<ChildB> a { get; set; }
}

But, this code will not compile. The error is:

`MyApp.Data.ParentA` does not implement interface member `MyApp.Data.IParent.a`.
`MyApp.Data.ParentA.a` cannot implement `MyApp.Data.IParent.a` because it does not have
the matching return type of `System.Collections.Generic.List<MyApp.Data.IChild>`.

Solution 1

Make IParent generic:

public interface IChild
{
}

public interface IParent<TChild> where TChild : IChild
{
    List<TChild> a { get; set; } 
}

public class ChildA : IChild {  }   

public class ChildB : IChild {  }   

public class ParentA : IParent<ChildA>
{
    public List<ChildA> a { get; set; }
}

public class ParentB : IParent<ChildB>
{
    public List<ChildB> a { get; set; }
}

Solution 2

You need to have the classes return a List<IChild>:

public class ParentA : IParent
{ 
    public List<IChild> a { get; set; }
}

public class ParentB : IParent
{ 
    public List<IChild> a { get; set; }
}

Solution 3

The implementation can only return List of IChild as follows:

public interface IChild
{
}

public interface IParent
{
    List<IChild> Children { get; set; }
}

public class ChildA : IChild
{
}

public class ChildB : IChild
{
}

public class ParentA : IParent
{

    public List<IChild> Children
    {
        get;
        set;

    }
}

public class ParentB : IParent
{
    public List<IChild> Children
    {
        get;
        set;
    }
}

Comments

[Jodrell]

It would be wiser to declare your List properties as ILists, they would be less retrictive but offer the same functionality. Doesn't help with the question though.

[nawfal]

then he can make it just IEnumerable. That may not be the requirement

[Suncat2000]

@Jodrell IList does not offer the same functionality as List.

[Jodrell]

@Suncat2000, you are correct but, IList, ICollection (thier readonly counterparts) or IEnumerable might be a smaller promise to make that does meet the required level of functionality.

[Rawling]

Not sure why this was up-voted. This won't compile. You can't convert a List<ChildX> to or from a List<IChild>.

[Rawling]

Do you mean, change the property declarations on ParentA and ParentB? The one on IParent is already a List<IChild>.

[mclark1129]

Yes, you will have to change the declarations on your implementing classes.

[levininja]

I really wish I could upvote this 100 times...this is a case study on why generics are so useful.

[OfirD]

Can you elaborate about the rationale behind the solution?