using System;

using Client;

using Server;

public class Program

{

    public static void Main()

    {

        IService<Callback, Proxy> test1 = new Service(); //possible

        IService<Callback, IProxy> test2 = new Service(); //possible

        IService<ICallback, IProxy> test3 = new Service(); //not possible

        IService<ICallback, Proxy> test4 = new Service(); //not possible

    }

}

​

namespace Server

{

    interface IService<in TCallback, out TProxy> { }

​

    interface ICallback { } //or class

    interface IProxy { } //or class

}

​

namespace Client

{

    using Server;

    class Callback : ICallback { }

    class Proxy : IProxy { }

​

    class Service : IService<Callback, Proxy>  { }

}

​