using System;

using System.Collections.Generic;

using System.Linq;

​

public class SO_Concat

{    

    public class A : Super    {    }

    public class Super

    {

        public string foo { get; set; }

    }

    public static void Main(string[] args)

    {

        var listA = new List<A>{new A{foo = "toto"}};

        var listSuper = new List<Super>{new Super{foo = "bar"}};

        //oneWay

        var bothSuper = listA.Concat(listSuper).ToList();

        bothSuper.Dump();

        //the other

        var bothA = listSuper.Concat(listA).ToList();

        bothA.Dump();

    }

}