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 Thing("thing 1", 10, 2), new Thing("thing 2", 20, 3)};

        IVisitor calc = new Calculate();

        foreach(var t in things) {

            t.Accept(calc);

            Console.WriteLine("{0} - Total price = {1} - Total with tax = {2}", t.name, t.total, t.totalWithTax);

        }

    }

    public interface IVisitor {

        void Visit(IThing thing);   

    }

    public interface IThing {

        void Accept(IVisitor visitor);  

    }

    public class Thing: IThing {

        public string name;

        public int units;

        public decimal unitPrice;

        public decimal total;

        public decimal totalWithTax;

        public Thing(string name, int units, decimal unitPrice) {

            this.name = name;

            this.unitPrice = unitPrice;

            this.units = units;

        }

        public void Accept(IVisitor visitor){

            visitor.Visit(this);

        }

    }

    public class Calculate : IVisitor

    {

        public void Visit(IThing thing){

            var _thing = (Thing)thing;

            _thing.total = _thing.units * _thing.unitPrice;

            _thing.totalWithTax = _thing.total * TAX;

        }

​

    }

}