How can we check whether a number is prime or not

Hi Naresh, Please follow the below link 

www.javatpoint.com/prime-number-program-in-csharp

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;
            int count = 0;
            Console.Write("Enter a number to check if number is prime or not :");
            num = int.Parse(Console.ReadLine());
            if(num <= 1)
            {
                Console.WriteLine("\nInvalid number please Try Again");
            }
            for(int i = 2; i < num/2; i++)
            {
                if(num % i == 0)
                {
                    count = 1;
                    break;
                }
            }
            if(count==1)
            {
                Console.WriteLine("\nEntered Number is Not Prime");
            }
            else
            {
                Console.WriteLine("\nEntered Number is Prime");
            }
            Console.ReadLine();
        }
    }
}

You can check the no. Is prime or not using below code in c#.

using System;
namespace Demo {
   class MyApp {
      public static void Main() {
         int n = 5, a = 0;
         for (int i = 1; i <= n; i++) {
            if (n % i == 0) {
               a++;
            }
         }
         if (a == 2) {
            Console.WriteLine("{0} is a Prime Number", n);
         } else {
            Console.WriteLine("Not a Prime Number");
         }
         Console.ReadLine();
      }
   }
}
Output
5 is a Prime Number