PriceCalculator.cs

namespace PriceCalculator
{
    using System;

    public static class Program
    {
        public static void Main()
        {
            do
            {
                bool exited;

                var price = GetPrice(out exited);

                if (exited)
                {
                    break;
                }

                var items = GetNumberOfItems(out exited);

                if (exited)
                {
                    break;
                }

                var payment = GetPayment(out exited);

                if (exited)
                {
                    break;
                }

                var change = payment - (price * items);

                Console.WriteLine("Change: {0}", change);
            }
            while (true);

            Console.WriteLine("Please come again!");
            Console.ReadLine();
        }

        private static T Get<T>(string name, Func<string, Tuple<bool, T>> parse, out bool exited)
        {
            string input;

            do
            {
                Console.Write("{0}: ", name);
                input = Console.ReadLine() ?? string.Empty;

                var result = parse(input);

                if (result.Item1)
                {
                    exited = false;

                    return result.Item2;
                }

                if (!"quit".Equals(input))
                {
                    break;
                }

                Console.WriteLine("Invalid number");
            }
            while (!"quit".Equals(input));

            exited = true;

            return default(T);
        }

        private static double GetPrice(out bool exited)
        {
            Func<string, Tuple<bool, double>> parse = s =>
            {
                double number;

                var result = double.TryParse(s, out number);

                return new Tuple<bool, double>(result, number);
            };

            return Get("Price", parse, out exited);
        }

        private static int GetNumberOfItems(out bool exited)
        {
            Func<string, Tuple<bool, int>> parse = s =>
            {
                int number;

                var result = int.TryParse(s, out number);

                return new Tuple<bool, int>(result, number);
            };

            return Get("Items", parse, out exited);
        }

        private static double GetPayment(out bool exited)
        {
            Func<string, Tuple<bool, double>> parse = s =>
            {
                double number;

                var result = double.TryParse(s, out number);

                return new Tuple<bool, double>(result, number);
            };

            return Get("Payment", parse, out exited);
        }
    }
}