DynamicDuckTyping.cs

using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            WhosThere(new classA());
            WhosThere(new classB());

            Console.ReadKey();
        }

        static void WhosThere(dynamic someone)
        {
            Console.WriteLine(someone.ItsMe());
        }
    }

    class classA
    {
        public string ItsMe() => "class A";
    }

    class classB
    {
        public string ItsMe() => "class B";
    }
}