(一)if语句
If语句是一个条件分支语句。
C#中，空语句是合法的，他不做任何操作。
好的编码风格要求语句块中要有缩进。
I f语句中if后面可以没有else，可以有多个else if，但只能最多有一个else！
例题：
/*if语句的简单应用
收入和逃税问题
double money;
Console.Write("月收入");
money = Convert.ToDouble(Console.ReadLine());
if (money > 3500)
{
Console.WriteLine("交税");
}
else
{
Console.WriteLine("逃税");
}
Console.WriteLine("恭喜");
学习断点（ 工具 选项 编辑 文本行号 调试）
/*男女身高体重健康值信息查看
Console.Write("性别：");
string sex = Console.ReadLine();
Console.Write("身高：");
int h =Convert.ToInt32 ( Console.ReadLine());
Console.Write("体重：");
int w = Convert.ToInt32(Console.ReadLine());
int m = 0, n = 0;
if(sex=="男")
{
m=h-100;
if(m-w>=-3&&m-w<=3)
{
Console.WriteLine ("标重");
}
else if(m-w<-3)
{
Console .WriteLine ("您胖了");
}
else
{
Console .WriteLine ("您瘦了");
}
}
else if(sex=="女")
{
n=h-110;
if(n-w>=-3&&n-w<=3)
{
Console.WriteLine ("标重");
}
else if(n-w<-3)
{
Console .WriteLine ("您胖了");
}
else
{
Console .WriteLine ("您瘦了");
}
}
else
{
Console .WriteLine ("你输入错误");
}
*/
*/
求一元二次方程根的情况
/* Console.WriteLine("求一元二次方程a*x*x+b*x+c=0根的情况");
Console.Write("a=");
int a=Convert .ToInt32 (Console .ReadLine());
Console.Write("b=");
int b= Convert.ToInt32(Console.ReadLine());
Console.Write("c=");
int c = Convert.ToInt32(Console.ReadLine());
if (a == 0)
{
Console.WriteLine("不是一元二次方程");
}
else
{
Console.WriteLine("一元二次方程");
int delta = b * b - 4 * a * c;
if(delta>0)
{
Console.WriteLine("两个不等的实数根");
}
else if (delta == 0)
{
Console.WriteLine("两个相等的实数根");
}
else
{
Console.WriteLine("无实数根");
}
}*/
房子 钱 能力 的比较
/*
错误源分析：read改为readline
Console.WriteLine("请输入三样东西");
Console.WriteLine("你有房吗?");
bool house = Convert.ToBoolean(Console.Read());
Console.WriteLine("你有钱吗?");
bool money = Convert.ToBoolean(Console.Read());
Console.WriteLine("你有能力吗?");
bool ability = Convert.ToBoolean(Console.Read());
if (house == true)
{
Console.WriteLine("马上结婚");
}
else if (money == true)
{
Console.WriteLine("先买房再结婚");
}
else if (ability == true)
{
Console.WriteLine("先挣钱在买房在结婚");
}
else
Console.WriteLine("拜拜");
*/
/*语句错误源分析 因未加中阔号引起
注意else后边加中括号
Console.WriteLine("请输入三样东西");
Console.Write ("你有房子吗?");
bool house = Console.ReadLine() == "有";
if (house == true)
{
Console.WriteLine("马上结婚");
}
else
Console.Write("你有钱吗?");
bool money=Console.ReadLine()=="有";
if (money == true)
{
Console.WriteLine("先买房在结婚");
}
else
Console.Write("你有能力吗?");
bool ability = Console.ReadLine() == "有";
if (ability == true)
{
Console.WriteLine("先挣钱在买房在结婚");
}
else
Console.WriteLine("拜拜");
*/
/*
房 钱 能力 正确版本 此版本全部回答后作出分析
Console.WriteLine("请输入三样东西");
Console.WriteLine("你有房吗?");
bool house = Convert.ToBoolean(Console.ReadLine());
Console.WriteLine("你有钱吗?");
bool money = Convert.ToBoolean(Console.ReadLine());
Console.WriteLine("你有能力吗?");
bool ability = Convert.ToBoolean(Console.ReadLine());
if (house == true)
{
Console.WriteLine("马上结婚");
}
else if (money == true)
{
Console.WriteLine("先买房再结婚");
}
else if (ability == true)
{
Console.WriteLine("先挣钱在买房在结婚");
}
else
{
Console.WriteLine("拜拜");
}
*/
/*
此版本更新创意
Console.WriteLine("请输入三样东西");
Console.WriteLine("你有房吗?");
bool house = Console.ReadLine()=="有";
Console.WriteLine("你有钱吗?");
bool money = Console.ReadLine()=="有";
Console.WriteLine("你有能力吗?");
bool ability =Console.ReadLine()=="有";
if (house == true)
{
Console.WriteLine("马上结婚");
}
else if (money == true)
{
Console.WriteLine("先买房再结婚");
}
else if (ability == true)
{
Console.WriteLine("先挣钱在买房在结婚");
}
else
{
Console.WriteLine("拜拜");
}
*/
/*此语句不需要全部回答问题，找到有效条件即可停止
Console.WriteLine("请输入三样东西");
Console.Write("你有房子吗?");
bool house = Console.ReadLine() == "有";
if (house == true)
{
Console.WriteLine("马上结婚");
}
else
{
Console.Write("你有钱吗?");
bool money = Console.ReadLine() == "有";
if (money == true)
{
Console.WriteLine("先买房在结婚");
}
else
{
Console.Write("你有能力吗?");
bool ability = Console.ReadLine() == "有";
if (ability == true)
{
Console.WriteLine("先挣钱在买房在结婚");
}
else
{ Console.WriteLine("拜拜"); }
}
}
*/
单机猜拳游戏
/* Console.WriteLine("单机猜拳游戏");
int 剪刀 = 0, 石头 = 1, 布 = 2;
int m=剪刀,n=石头,q=布;
Random b = new Random();
int bb = b.Next(3);
int aa = Convert.ToInt32(Console .ReadLine ());
if (aa == 0&&bb==0)
{
Console.WriteLine("人:"+m);
Console.WriteLine("电脑:"+m);
Console .WriteLine("平");
}
else if (aa == 0 && bb == 1)
{
Console.WriteLine("人:" + m);
Console.WriteLine("电脑:" +n);
Console.WriteLine("输");
}
else if (aa == 0 && bb == 2)
{
Console.WriteLine("人:" + m);
Console.WriteLine("电脑:" + q);
Console.WriteLine("赢");
}
else if (aa == 1 && bb == 0)
{
Console.WriteLine("人:" + n);
Console.WriteLine("电脑:" + m);
Console.WriteLine("赢");
}
else if (aa == 1 && bb == 1)
{
Console.WriteLine("人:" + n);
Console.WriteLine("电脑:" +n);
Console.WriteLine("平");
}
else if (aa == 1 && bb == 2)
{
Console.WriteLine("人:" +n);
Console.WriteLine("电脑:" + q);
Console.WriteLine("输");
} else if (aa == 2 && bb == 0)
{
Console.WriteLine("人:" +q);
Console.WriteLine("电脑:" + m);
Console.WriteLine("输");
}
else if (aa == 2 && bb == 1)
{
Console.WriteLine("人:" + q);
Console.WriteLine("电脑:" + n);
Console.WriteLine("赢");
}
else if (aa == 2 && bb == 2)
{
Console.WriteLine("人:" + q);
Console.WriteLine("电脑:" + q);
Console.WriteLine("平");
}
else {
Console.WriteLine("无意义");
}
*/
//for循环语句应用
//穷举法包括全部显示和用条件筛选出合适的值，迭代法求和的
//(1初始条件；2循环条件；4状态改变)
// {
// 3循环体
//}
// for (int i = 0; i < 128; i++)
//{
// Console.WriteLine(i);
// Console.Write(i+"\t");
// Console.Write((char)i + "\t");
//Console.Write(i+"="+(char)i + "\t");
/*
if(i>2)
{
Console.Write(i+"="+(char)i + "\t");
}
*/
// }
/* for (int i = 1; i <= 100; i++)
{
// if(i%2==1)一百内的所有奇数
// if(i%7==0||i%10==7||i/10==7)
{
Console .Write (i);
}
}*/
/*
int sum=0;
for (int i = 1; i <= 100; i++)
{
sum = sum + i;
}
Console.WriteLine(sum);求一到一百的和
*/
/*
int sum = 0;
for (int i = 1; i <= 100; i++)
{
sum = sum + i;
Console.WriteLine(sum);显示一到一百求和的每个过程值
}
*/
/*
int n = 1;
for (int i = 1; i <= 100; i++)
{
n = n + 2;
}
Console.WriteLine(n);
*/
//一个猴子，一堆桃，猴子每天吃总数的一半,然后从剩下的扔一个坏的，到第七天早上只有一个桃子，求总数
/*
int n=1;
for (int i = 1; i <= 6; i++)
{
n=2*(n+1);
}
Console.WriteLine(n);
*/
//一对兔子开始为幼兔，第二个月为未成年，第三个月为成年兔，成年兔从第三个月开始每月生育一对兔子。
//问开始有一对，24月有多少对？解析：第n+2个月增加的对数等于第n个月的对数！所以每个月对数等于前两月之和
/*
int tu1 = 1, tu2 = 1, tu=0;
for (int i = 3; i <= 24; i++)
{
tu = tu1 + tu2;
tu2 = tu1;
tu1 = tu;
}
Console.WriteLine(tu);
*/
// 长方形
/*
for(int i=1;i<=5;i++)
{
for (int n = 1; n <= 10; n++)
{
Console.Write( "★");
}
Console.Write("\n");
}
*/
/*
★
★★
★★★
for (int i = 1; i <= 5; i++)
{
for (int n = 1; n <= i; n++)
{
Console.Write("★");
}
Console.Write("\n");
}
*/
/*
★★★
★★
★
for (int i = 1; i <= 5; i++)
{
for (int n = 1; n <= 6-i; n++)
{
Console.Write("★");
}
Console.Write("\n");
}
*/
/*
★★★
★★
★
for (int i = 1; i <= 5; i++)
{
for (int n = 5; n >= i; n--)
{
Console.Write("★");
}
Console.Write("\n");
}
*/
// 小九九算数
/*
for (int i = 1; i <= 9; i++)
{
for (int n = 1; n <= i; n++)
{
Console.Write(i+"*"+n+"="+i*n);
}
Console.Write("\n");
}
*/
/*
★注意：一个五角星占两个空格
★★
★★★
for (int i = 1; i <= 5; i++)
{
for (int n = 1; n <= 5 - i; n++)
{
Console.Write(" ");
}
for (int m = 1; m <= i;m++ )
{
Console.Write("★");
}
Console.Write("\n");
}
*/
/*
★★★
★★
★
for (int i = 1; i <= 5; i++)
{
for (int n = 1; n < i; n++)
{
Console.Write(" ");
}
for (int m = 1; m <= 6-i; m++)
{
Console.Write("★");
}
Console.Write("\n");
}
*/
/*
★
★★★
★★★★★
for (int i = 1; i <= 5; i++)
{
for (int n = 1; n <=6- i; n++)
{
Console.Write(" ");
}
for (int m = 1; m <i*2; m++)
{
Console.Write("★");
}
Console.Write("\n");
}
*/
/*
★
★★★
★★★★★
★★★
★
for (int i = 1; i <= 9; i++)
{
if (i <= 5)
{
for (int n = 1; n <= 5 - i && i <= 5; n++)
{
Console.Write(" ");
}
for (int m = 1; m < i * 2 && i <= 5; m++)
{
Console.Write("★");
}
}
else
{
for (int q = 6; q <= i; q++)
{
Console.Write(" ");
}
for (int w = 6; w < 25 - 2 * i ; w++)
{
Console.Write("★");
}
}
Console.Write("\n");
}
*/
原文：http://www.cnblogs.com/wanwuguizong/p/3921187.html