using System;

public class Program

{

    public static void Main()

    {

        string userInput;

        double diameter;

        Console.Write("\nPlease enter the diameter of your pizza: ");

        userInput = Console.ReadLine();

        while(true)

        {

            var isDouble = double.TryParse(userInput, out diameter) == true;

​

            if (!isDouble)

            {

                Console.WriteLine("\nENTRY NON-NUMBERIC ERROR\n");

                Console.WriteLine("Pizza must have a numeric diameter. You entered: \"{0}\"\n", userInput);

                Console.WriteLine("Please try again.\n");

                diameter = 0;

                //**if userInput isn't valid, diameter becomes 0**

            }

            else

            {

                if (diameter == 0)

                {

                    Console.Write("\n Exit initiated");

                    break;

                }

            }

            Console.Write("\nPlease enter the diameter of your pizza (0 to end program): ");

            userInput = Console.ReadLine();

            double.TryParse(userInput, out diameter);

            Console.Clear();

        }

    }

}