using System;

using System.Collections.Generic;

public class Program

{

    public const decimal TAX = 1.1m;

    public static void Main()

    {

        List<Thing> things = new List<Thing>{ 

            new ThingSingle{ Name = "thing 1", UnitPrice = 2m}, 

            new ThingMultiple { Name = "thing 2", Units = 10, UnitPrice = 3m},

            new ThingMultiple { Name = "thing 3", Units = 20, UnitPrice = 5m}

        };

        IVisitor calc = new Calculate();

        foreach(var t in things) 

        {

            t.Accept(calc);

            if(t is ThingSingle)

                Console.WriteLine("{0} - Total with tax = {1}", t.Name, t.TotalWithTax);

            else

                Console.WriteLine("{0} - Units = {1} - Total price = {2} - Total with tax = {3}", t.Name, t.Units, t.Total, t.TotalWithTax);

        }

    }

    public interface IVisitor {

        void Visit(Thing thing);    

    }

    public abstract class Thing {

        public string Name {get; set;}

        public virtual int Units {get; set;}

        public decimal UnitPrice {get; set;}

        public virtual decimal Total {get; set;}

        public decimal TotalWithTax {get; set;}

        public virtual void Accept(IVisitor visitor){

            visitor.Visit(this);

        }

    }

    public class ThingSingle: Thing {

        public override int Units {get {return 1;}}

        public override decimal Total {get { return UnitPrice; }}

    }

    public class ThingMultiple: Thing {

    }

    public class Calculate : IVisitor

    {

        public void Visit(Thing thing){

            if(thing is ThingMultiple)

                thing.Total = thing.Units * thing.UnitPrice;

            thing.TotalWithTax = thing.Total * TAX;

        }

    }

}