using System;

using System.Collections.Generic;

using System.Linq;

class Program

{

    static void Main(string[] args)

    {

    }

}

​

public interface IA1

{

    long Id { get; set; }

}

​

public interface IA2 : IA1

{

}

​

public interface IA3 : IA2

{

}

​

public interface IB1

{

    Guid Id { get; set; }

}

​

public interface IB2 : IB1

{

}

​

public interface IB3 : IB2

{

}

​

public class C1

{

    //public IEnumerable<IA2> F1()

    //{

    //    var a = new List<IA3>();

    //    return a;

    //}

    //public IEnumerable<IB2> F2()

    //{

    //    var b = new List<IB3>();

    //    return b;

    //}

    public IEnumerable<IA2> F3()

    {

        var a = new List<IA3>();

        var a1 = new List<IA3>();

        var b = new List<IB3>();

        return a.Concat(a1).Concat(b);

    }

}