How to check a number Prime or not in c# ?

/*Check a number Prime or not in C# */

Program :

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace Primenumber

{

    class Program

    {

        static void Main(string[] args)

        {

            int num, flag = 0;

            Console.WriteLine("Enter A number :");

            num = Convert.ToInt32(Console.ReadLine());

            if (num > 0)

            {

                for (int i = 2; i < num - 1; i++)

                {

                    if (num % i == 0)

                    {

                        flag = 1;

                        break;

                    }

                }

                if (flag == 0)

                    Console.WriteLine("The number " + num + " is a Prime number .");

                else

                    Console.WriteLine("The number " + num + " is not a Prime number .");

            }

            else

                Console.WriteLine("Enter A valid Number :\n");

            Console.ReadKey();

        }

    }

}

Output :

Enter A number :

5

The number 5 is a Prime number .

           

             

Enter A number :

6

The number 6 is not a Prime number .