控制台程序

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

namespace test3_6
{
    class Program
    {
        static void Main(string[] args)
        {
            int sum = 0;
            do
            {
                Console.WriteLine("请输入一个正整数计算它的阶乘，输入一个负整数结束程序");
                Console.Write("请输入：");
                sum = Convert.ToInt32(Console.ReadLine());
                if (sum >= 0)
                {
                    long jc = 1;
                    for (int i = 1; i <= sum; i++)
                    {
                        jc *= i;
                    }
                    Console.WriteLine(sum.ToString() + "的阶乘=" + jc.ToString());
                    Console.WriteLine();
                }
            } while (sum >= 0);
        }
    }
}