using System;

public static class Program

{

        //extension methods must be static

        //the this keyword tells C# that this is an extension method

        public static bool IsPrime(this int number)

        {

            //check for evenness

            if (number % 2 == 0)

            {

                if (number == 2)

                    return true;

                return false;

            }

            //you don’t need to check past the square root

            int max = (int)Math.Sqrt(number);

            for (int i = 3; i <= max; i += 2)

            {

                if ((number % i) == 0)

                {

                    return false;

                }

            }

            return true;

        }

    public static void Main()

    {

            int k = 0;

            SimpleMethod(ref k);

            Console.WriteLine(k);

            int Sum = 0;

            int Product=0;

            Calculate(100,200,out Sum, out Product );

            Console.WriteLine("Sum = {0} && Product = {1}", Sum, Product);

            int[] Numbers = new int[3];

            Numbers[0] = 100;

            Numbers[1] = 200;

            Numbers[2] = 300;

            ParamsMethod(Numbers);

        for (int i = 0; i < 20; ++i)

        {

            if (i.IsPrime())

            {

                Console.WriteLine(i);

            }

        }

    }

    public static void SimpleMethod(ref int j)

    {

      j= 500;

    }

    public static void Calculate(int a,int b, out int sum, out int product)

    {

      sum = a + b;

      product = a * b;

    }

    public static void ParamsMethod(params int[] numbers)

    {

        Console.WriteLine("There are {0} elements", numbers.Length);

        foreach (int i in numbers )

        {

            Console.WriteLine(i);

        }

    }

}