Call C# from C++ in a windows phone 8.0 solution

Question

I am trying to call C# code in a Cocos2d-x application (windows phone part).

I have add to my solution a Windows Phone Runtime Component to do this but in my C# project, i cannot implement an interface which is define the Windows Runtime Component project.

I followed this thread without success : Calling C# method from C++ code in WP8

Some code of my solution :

The interface in the windows phone runtime component which i'm trying to implement through a c# class

namespace WindowsPhoneRuntimeComponent
{
    [Windows::Foundation::Metadata::WebHostHidden]
    public interface class ICallback
    {
    public:
         virtual void Exec( Platform::String ^Command, Platform::String ^Param);
    };
}

The c# class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Runtime;
using WindowsPhoneRuntimeComponent;

namespace cocos2d
{
    class CallbackImpl : ICallback // Here is the problem
    {

    }
}

In the C# class from the C# project, i can reference the windows runtime component calling "using WindowsPhoneRuntimeComponent;" but when i trying to call something from the namespace, or when i type "WindowsPhoneRuntimeComponent::something" there is nothing after "::".