1、用 if……else 判断是否为闰年（闰年为能被4整除的年份，*注：若该年份能被100整除，还要被400整除才是闰年）

Console.WriteLine("请输入年份:");int year = int.Parse(Console.ReadLine());if (year % 4 == 0){    if (year % 100 == 0)    {        if (year % 400 == 0)        {            Console.WriteLine("yes");        }        else            Console.WriteLine("no");    }    else        Console.WriteLine("yes");}else    Console.WriteLine("no");

2、if……else if……else 输入一个1~100之内的数，判断是否与7有关

Console.WriteLine("请输入一个1~100内的数");int shu = int.Parse(Console.ReadLine());if (shu >= 1 && shu <= 100){    if (shu%10==7)        Console.WriteLine("yes");    else if(shu%7==0)            Console.WriteLine("yes");    else if(shu/10==7)            Console.WriteLine("yes");    else            Console.WriteLine("no");}else{    Console.WriteLine("输入错误");}

 3、输入a,b,c，求一元二次方程的解

Console.WriteLine("请输入a,b,c");            int a  = int.Parse(Console.ReadLine());            int b  = int.Parse(Console.ReadLine());            int c  = int.Parse(Console.ReadLine());            if (a == 0 && b != 0)            {                double x =(double) (-c) / b;                Console.WriteLine("有一个实根{0}",x);            }            else if (a == 0 && b == 0)            {                Console.WriteLine("a，b均为零，无实根");            }            else            {                double de =sqr( b * b - 4 * a * c);                if (de == 0)                {                    double x = (double)(-b) / (2 * a);                    Console.WriteLine("有两个相等的实根，x1=x2={0}", x);                }                else if (de < 0)                {                    Console.WriteLine("此方程无解");                }                else                {                    double x1 = (double)((-b) + Math.Sqrt(de)) / (2 * a);                    double x2 = (double)((-b) - Math.Sqrt(de)) / (2 * a);                    Console.WriteLine("有两个实根,x1={0},x2={1}",x1,x2);                }            }