循环

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

namespace _xxxx
{
    class Program
    {
        static void Main(string[] args)
        {
            /*int t = 0;//计数器
            for (int i = 0; i <= 200; i++)
            {
                if (i % 7 == 0 && i % 4 != 0)
                {
                    Console.Write(i + "\t");
                                                                                        t++;
                    if (t % 6 == 0)
                        Console.WriteLine();
                }
            }
            int i;//循环控制变量
            double score;
            double sum=0;
            Console.WriteLine("请输入10个学生的成绩");
            for (i = 0; i < 10;i++ )
            {//重复输入
                score = Convert.ToDouble(Console.ReadLine());
                sum = sum + score;
                  
            }
            Console.WriteLine("这些学生的平均分为"+sum/(i-1));
            char an;
            Console.WriteLine("练习的怎么样（y/n）");
            an = Convert.ToChar(Console.ReadLine());
            while(an!='y')
            {
                Console.WriteLine("早上练习");
                Console.WriteLine("上午练习");
                Console.WriteLine("下午练习");
                Console.WriteLine("晚上练习");
                an = Convert.ToChar(Console.ReadLine());
            }
            Console.WriteLine("你好棒");*/
            string an;
            Console.WriteLine("请输入密码");
            an =(Console.ReadLine());
            while (an != "jzit")
            {
                Console.WriteLine("请输入密码");
                an = (Console.ReadLine());
            }
            Console.WriteLine("-欢迎进入系统-");
        }
    }
}

 一个循环的案例

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            //int n;
            //int sum = 0;
            //Console.WriteLine("请输入n的值");
            //n = Convert.ToInt32(Console.ReadLine());
            //for (int i = n; i <= 10; i++)
            //{
            //    sum = sum + i;

            //}
            //Console.WriteLine("若干个数的和为" + sum);
            //int i = n;
            //do
            //{
            //    sum = sum + i;
            //    i++;
            //}
            //while (i <= 10);
            //Console.WriteLine("若干个数的和为"+sum);
            //while (i <= 10)
            //{
            //    sum = sum + i;
            //    i++;
            //}
            //Console.WriteLine("若干个数的和为" + sum);
            //string password;
            //Console.WriteLine("请输入密码");
            //password = Console.ReadLine();
            //while (password != "jzit")
            //{
            //    Console.WriteLine("请输入正确密码");
            //    password = Console.ReadLine();
            //}
            //Console.WriteLine("--------------");
            //Console.WriteLine("*欢迎进入系统*");
            //Console.WriteLine("--------------");
            //double shu, sum = 0;
            //Console.WriteLine("请输入数");
            //shu = Convert.ToDouble(Console.ReadLine());
            //while (shu != -1)
            //{
            //    sum = sum + shu;
            //    Console.WriteLine("请继续输入数");
            //    shu = Convert.ToDouble(Console.ReadLine());
            //}
            //Console.WriteLine("这几个数的和为" + sum);
            //int n;
            //for (n = 1; n <= 100; n++)
            //{
            //    if(n % 3 == 0 && n % 8 == 0) continue;
            //    Console.Write(n + "\t");
            //}
            
            
            string password;
            int i;
            Console.WriteLine("请输入密码");
            password = Console.ReadLine();
            
            for (i = 2; i > -1; i--)
            {
                if (password == "12345")
                {
                    Console.WriteLine("--------------");
                    Console.WriteLine("-欢迎进入系统-");
                    Console.WriteLine("--------------"); break;
                }
                if (i == 0)
                { Console.WriteLine("对不起，你已经没有机会"); break; }
                else
                {
                    Console.WriteLine("密码输入错误，你还有" + i + "次机会，请重新输入");
                    password = Console.ReadLine();
                }
                
            }
            
        }
    }
}