using System;

using System.Threading.Tasks;

​

namespace AsyncTest

{

    public class Program

    {

        public static bool IsPrime(int number)

        {

            if (number == 1) return false;

            if (number == 2) return true;

            if (number % 2 == 0)  return false;

            var boundary = (int)Math.Floor(Math.Sqrt(number));

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

            {

                if (number % i == 0)  return false;

            }

            return true;        

        }

        public static int GetPrimeByIndex(int index)

        {

            var i = 2;

            while (index > 0)

            {

                i++;

                if (IsPrime(i)) index--;                

            }

            return i;

        }

        public static void Main(string[] args)

        {

            var prime = GetPrimeByIndex(3);

            Console.WriteLine($"result = {prime}");

        }

    }

}

​