using System;

using System.Collections.Generic;

public class Program

{

    class A 

    {

        public virtual void Do() 

        {

            Console.WriteLine("A.Do()");

        }

    }

    class B : A 

    {

        public override void Do() 

        {

            Console.WriteLine("B.Do()");

        }

    }

    public static void Main()

    {

        IEnumerable<A> enumerable = 

            new B[] { 

            new B(), 

            new B(), 

            new B() 

        };

        IList<B> list = (IList<B>) enumerable;

        list[0].Do();

    }

}

​