using System;

public class Program

{

    public static void Main()

    {

        Base[] array = new Base[2];

        array[0] = new Derived1();  // Derived1 goes in here.

        array[1] = new Derived2();  // Derived2 goes in here.

        foreach (Base b in array)

                Console.WriteLine(b.Foo() + " "  + b.test() + " " + b.Number.ToString());

        IUsable[] array02 = new IUsable[1];

        array02[0] = new Derived2();

        Console.WriteLine(array02[0].Usable("test"));

    }

    public abstract class Base

    {

        public abstract string Foo();

        public virtual string test() {

            return "test";

        }

        private int number = 0;

        public int Number { get; set; }

        public Base() {

            this.Number = 5;

        }

    }

​

    public class Derived1: Base

    {

        public override string Foo()

        {

            return "Derived1";

        }

        public override string test() {

            return "test of d1";

        }

        public string own() 

        { return "my own output"; }

    }

​

    public class Derived2: Base, IUsable 

    {

        public override string Foo()

        {

            return "Derived2";

        }

        public string Usable(string input) {

            return input + " this has been modified";

        }

    }

    public interface IUsable 

    {

        string Usable(string input);  

    }

}