using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

​

namespace Switches

{

    class Program

    {

        static void Main(string[] args)

        {

            double no1 = 0;

            double no2 = 0;

            string op = "";

            string answer = "";

            Console.WriteLine("Welcome to the basic calculator.");

            EnterNo1:

                /*Prompt user to enter 1st no, validate no &

capture value in no1 variable*/

                Console.WriteLine("Please enter a valid number:");

            if (!double.TryParse(Console.ReadLine().Trim(), out no1))

            {

                goto EnterNo1;

            }

​

            EnterOperator:

                /*Prompt user to enter an operator, validate

operator and capture value in op variable*/

                Console.WriteLine("Please enter one of the following operators: +, -, *, /");

            op = Console.ReadLine().Trim();

            if (op != "+" && op != "-" && op != "*" && op != "/")

            {

                goto EnterOperator;

            }

​

            EnterNo2:

                Console.WriteLine("Please enter a valid 2nd number:");

            if (!double.TryParse(Console.ReadLine().Trim(), out no2))

            {

                goto EnterNo2;

            }

​

            //Calculate and output answer:

            answer = Calculate(no1, no2, op).ToString();

            Console.WriteLine($"{no1} {op} {no2} = {answer}.");

            Console.ReadKey();

        }

​

         double Calculate(double no1, double no2, string op)

        {

            double answer = 0;

            switch (op)

            {

                case "+":

                    answer = no1 + no2;

                    break;

                case "-":

                    answer = no1 - no2;

                    break;

                case "*":

                    answer = no1 * no2;

                    break;

                case "/":

                    answer = no1 / no2;

                    break;

            }

​

            return answer;

        }

    }

}