1、

Console.WriteLine("请您输入一个数字:");

int  i = int.Parse(Console.ReadLine());

if(i>=21)

Console.WriteLine(i);

else

continue;

2、Console.WriteLine("请输入要查找的文字")；

string inputstr = Console.ReadLind();

string[] mystr = new string[3];

mystr[0] = "星期一";

mystr[1] = "星期二";

mystr[2] = "星期三";

for(int i = 0;i<mystr.Length;i++)

{

if(mystr[i].Equals(inputstr))

{

goto Found;

}

}

Console.WriteLine("您查找的{0}不存在!",inputstr);

goto Finish;

Found:

Console.WriteLine("您查找的{0}存在!",inputstr);

Finish:

Console.WriteLine("查找完毕!");

Console.ReadLine();

3、Console.WriteLine("请您输入一个整数");

int MyInteger = int.Parse(Console.ReadLine());

int MyInteger;

string MyWeek;

switch(MyInteger){

case 1:  MyWeek = "您输入的整数属于周一!";break;

case 2:  MyWeek = "您输入的整数属于周二!";break;

case 3:  MyWeek = "您输入的整数属于周三!";break;

case 4:  MyWeek = "您输入的整数属于周四!";break;

case 5:  MyWeek = "您输入的整数属于周五!";break;

case 6:  MyWeek = "您输入的整数属于周六!";break;

case 7:  MyWeek = "您输入的整数属于周日!";break;

default:

MyWeek = "数字输入错误!";break;}Console.WriteLine(MyWeek);}