判断质数

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                Console.Write("请输入数字a=");
                int a = int.Parse(Console.ReadLine());
                int num = 0;
                for (int i = 1; i <= num; i++)
                {
                    double b = a % i;
                    if (b == 0)
                    {
                        num++;
                    }
                }
                if (num == 2)
                {
                    Console.WriteLine("你输入的{0}是质数", a);
                }
                else
                { 
                Console.WriteLine("你输入的{0}不是质数", a）;
                }

                Console.ReadLine();
            }
        }
    }
}