c#之函数

1.函数

1.函数

/// <summary>
/// main
/// 主函数（主方法）控制台的输出输入控制函数
/// </summary>
/// <param name="args">字符串参数</param>
static void Main(string[] args) //static 静态修饰符，void（int,string char）是无返回值+方法名（参数）{ 方法体}
{
int num1 = int.Parse(Console.ReadLine());
int num2 = int.Parse(Console.ReadLine());

string num3= GetMax(num1, num2);
Console.WriteLine(num3);


//Run();
//ChangeSelf();
//Run();
//ChangeSelf();
//int num1 = 10;
//Console.WriteLine(num1);
//return ; //结束当前方法
//int num2 = 20;
//Console.WriteLine(num2);

}
public static string GetMax(int num1,int num2) 
{

int num3 = num1 > num2 ? num1 : num2;
//Console.WriteLine(num3);
return num3+"";
}
public static void Run() {

Console.WriteLine("超级玛丽，跑啊跑，跳啊跳");
Console.WriteLine("超级玛丽，跑啊跑，跳啊跳");
Console.WriteLine("超级玛丽，跑啊跑，跳啊跳");
Console.WriteLine("超级玛丽，跑啊跑，跳啊跳");
Console.WriteLine("超级玛丽，跑啊跑，跳啊跳"); 
}
public static void ChangeSelf() {

Console.WriteLine("遇见一个砖块，撞一下无敌");
Console.WriteLine("屏幕闪烁，身体变大");
Console.WriteLine("遇到小怪，无敌消失");

}

例子1：
 public static int a = 3;
        static void Main(string[] args)//void 无返回值
        {
           // int a = 3;
            Text();
            Console.WriteLine(a);//作用域不同产生的结果不同
        }
        public static void Text() {
            a = a + 5;
        }

例子2：
 Console.WriteLine("请输入年份");
            string s = Console.ReadLine();
            year= IsInt(s);
           bool b= IsRun(year);

           Console.WriteLine("{0}是闰年吗？{1}", year, b);

        }
        public static int IsInt(string s) {
            int year;
            while (true)
            {
                try
                {
                    year = int.Parse(s);
                    return year;
                }
                catch 
                {
                    Console.WriteLine("您输入的有误，请在输入一次");
                    s = Console.ReadLine();
                }
            }
        
         
        }
        public static void Text()
        {
            a = a + 5;
        }
        public static bool IsRun(int year)
        {

            bool b = year % 400 == 0 || year % 4 == 0 && year % 100 != 0;
            return b;

        }

例子3：
//利用方法求两个数的和与积
            int num1 = 20;
            int num2 = 30;
          int[] s=  GetSum(52,63);//实参

          Console.WriteLine("和为{0}，积为{1}",s[0],s[1]);
        }
        public static int[] GetSum(int a, int b)//形参,变量名字与main的变量名字没有任何关系
        {
            //Console.WriteLine("和为" + (num1 + num2));
            //Console.WriteLine("积为" + (num1 * num2));
            int sum = a + b;
            int[] nums = { sum, (a * b) };
            return nums;
}

例子4：
int[] nums = {10,54,63,55,45,65,23,54,33,65,46 };
            int[] n=  GetResult(nums);
            foreach (var item in n)
            {
                Console.WriteLine(item);
            }
        }
        public static int[] GetResult(int[] nums)
        {
            int sum = 0;
            int avg = 0;
            int min = nums[1];
            int max=nums[5];
            for (int i = 0; i < nums.Length; i++)
            {
                if (nums[i]>max)
                {
                    max = nums[i];
                }
                if (nums[i]<min)
                {
                    min = nums[i];
                }
                sum += nums[i];
            }
            avg = sum / nums.Length;

            int[] n = { max,min,sum,avg};
            return n;
        
        }

using System;

namespace _002_函数
{
    class Program
    {
        static void Main(string[] args)
        {
            int year;
            Console.WriteLine("请输入年份");
            string s = Console.ReadLine();
            year = IsInt(s);
            bool b = IsRun(year);
            Console.WriteLine("{0}是闰年吗？{1}",year,b);
        }
        public static int IsInt(string s)
        {
            int year;
            while (true)
            {
                try
                {
                    year = int.Parse(s);
                    return year;
                }
                catch 
                {
                    Console.WriteLine("输入有误，请重新输入");
                    s = Console.ReadLine();
                }
            }
        }
        public static bool IsRun(int year)
        {
            bool b = year % 400 == 0 || year % 4 == 0 && year % 100 != 0;
            return b;
        }
    }
}

using System;

namespace _006_作业3
{
    class Program
    {
        static void Main(string[] args)
        {
         
            Console.WriteLine("请输入用户名和密码：");
            string name = Console.ReadLine();
            string mima = Console.ReadLine();
            int password = IsInt(mima);
            Name(name,password);
        }
        #region   判断密码
        public static int IsInt(string mima)
        {
            
            
            while (true)
            {
                int password = 0;
                try
                {
                    password = int.Parse(mima);
                    return password;
                }
                catch
                {
                    Console.WriteLine("密码输入有误，请重新输入");
                   
                     mima = Console.ReadLine();
                }
                
            }
             
        }
        #endregion
        #region      判断用户名和密码
        public static void Name(string name,int password)
        {
            int i = 1;
            while (i < 5)
            {
                if (name == "admin" && password == 123456)
                {
                    Console.WriteLine("登录成功");
                    break;
                }
                else
                {
                    Console.WriteLine("密码或用户名错误，请重新输入");
                    name = Console.ReadLine();
                    string mima = Console.ReadLine();
                    password = IsInt(mima);
                }
                i++;
            }
        }
        #endregion
    }
}

using System;

namespace _007_作业4_菲波那切数列
{
    class Program
    {
        static void Main(string[] args)
        {
            int a = 0;
            int b = 1;
            int index = SS(a, b);
            Console.WriteLine("斐波那数列第十个数为"+index);
    }
        public static int SS(int a,int b)
        {
            int c = 0;
                for (int i = 1; i < 9; i++)
                {
                c = a + b;
                a = b;
                b = c;
            }
            return c;
            }
        }
}

using System;

namespace _008_作业5_比较三个数大小
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("请输入三个数");
            int a = int.Parse(Console.ReadLine());
            int b = int.Parse(Console.ReadLine());
            int c = int.Parse(Console.ReadLine());
            IsInt(a, b, c);
        }
        public static void IsInt(int a,int b,int c)
        {
            int max = a > b ? a : b;
            max = max > c ? max : c;
            int min = a < b ? a : b;
            min = min < c ? min : c;
            int zj = (a + b + c) - max - min;
            Console.WriteLine("最大值为"+max+"中间值为"+zj+"最小值为"+min);
        }
    }
}

using System;

namespace _009_作业6_计算器
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("请输入要进行的计算，和数字");
           string c = Console.ReadLine();
            PanDuanFuHao(c);
        }
        #region    判断运算符
        public static void PanDuanFuHao(string c)
        {
            int a  = int.Parse(Console.ReadLine());
             
            int b = int.Parse(Console.ReadLine());

            switch (c)
            {
                case "+":
                    JF( a,b);
                    break;
                case "-":
                    JianF(a, b);
                    break;
                case "*":
                    CF(a, b);
                    break;
                case "/":
                    ChuF(a, b);
                    break;
                default:
                    Console.WriteLine("请重新输入");
                    break;
            }
            
        }
        #endregion
        #region     加
        public static void JF(int a,int b)
        {
            Console.WriteLine("和为"+(a+b));

        }
        #endregion
        #region     减
        public static void JianF(int a, int b)
        {
            Console.WriteLine("差为" + (a - b));

        }
        #endregion
        #region    乘
        public static void CF(int a, int b)
        {
            Console.WriteLine("积为" + (a * b));

        }
        #endregion
        #region  除
        public static void ChuF(int a, int b)
        {
            Console.WriteLine("商为" + (a / b));

        }
        #endregion
    }
}

2.

函数的重载
构成重载的条件：
1.方法名必须一样，参数列表不同（参数类型不同，参数个数不同）
2.与返回值无关

 static void Main(string[] args)
        {
            Sum(10,20);
                
            //方法构成重载的条件:1.方法名必须一样，参数列表不同（参数类型不同，参数个数不同）2.与返回值无关
            Console.WriteLine();

        }
        public static void Sum(int num1, int num2)
        {

            Console.WriteLine(num1 + num2);
        }
        public static int Sum(int num3, int num4,int num5)
        {
            return num3 + num4 ;
        }
        public static string Sum(int a, bool b) {
            return "";
        }
        public static void Sum(int num6, string num7) 
        {

            Console.WriteLine(num6+int.Parse(num7));
        
        }

using System;

namespace _014_函数的重载
{
    
    class Program
    {
        public static int MaxValue(params int[] arry)
        {
            int maxvalue = arry[0];
            for (int i = 1; i < arry.Length; i++)
            {
                if (arry[i] > maxvalue)
                {
                    maxvalue = arry[i];
                }
            }
            return maxvalue;
        }
        public static double MaxValue(params double[] arry)
        {
            double maxvalue = arry[0];
            for (int i = 1; i < arry.Length; i++)
            {
                if (arry[i] > maxvalue)
                {
                    maxvalue = arry[i];
                }
            }
            return maxvalue;
        }
        static void Main(string[] args)
        {
            int res = MaxValue(123, 25, 63);
            double res1 = MaxValue(3.14, 2.28, 56.3);
            Console.WriteLine(res1);
        }
    }
}

        
例子：ref
 static void Main(string[] args)
        {
            //ref
            int num = 20;
            int num1 = 30;
            int sum=0;
            Sum(ref  sum, num, num1);//ref:把实参中的值带入形参参与运算，运算完毕赋值再带出来(ref的值必须赋予初值)
            Sum(ref num,ref num1);
            Console.WriteLine(num+"  "+num1);
        }
        public static void Sum(ref int sum, int num, int num1)
        {
            sum = num + num1;
         
        }
        public static void Sum(ref int a,ref int b) 
        {
            int sum = a + b;
            a = a > b ? a : b;
            b = sum - a;
        
        
        }

using System;

namespace _015_ref
{
    class Program
    {
        static void Main(string[] args)
        {
            int num1 = 10;
            int num2 = 50;
            MaxValue(ref  num1,ref num2);//把实参的值带入形参参与运算，运算wanbi 赋值在带出来（ref值必须赋予初值）
            Console.WriteLine(num1+"    "+num2);
            
        }
        public static void MaxValue( ref int num1, ref int num2)
        {
            int sum = num1+num2;
            num1 = num1 > num2 ? num1 : num2;
            num2 = sum - num1;

        }
    }
}

例子：out
 Console.WriteLine("请输入数字");
            string num = Console.ReadLine();
            int number;
          bool b=  TryPase(num,out number);
          if (b)
          {
              Console.WriteLine(number);
          }
          else
          {
              Console.WriteLine(num);
          }

        }
        public static bool TryPase(string num,out int number) 
        {
            number = 0;
            try
            {
                number = int.Parse(num);
                return true;
            }
            catch 
            {

                return false;
            }
        
        
        }

using System;

namespace _016_out
{
    class Program
    {
        static void Main(string[] args)
        {
            int a = 12;
            int b = 2;
            Sum(out a, out b);
            Console.WriteLine(a + " " + b);
        }
        public static void Sum(out int a, out int b)
        {
            a = 100;
            b = 12;
            a = a + b;
        }
    }
}

       

using System;

namespace _018_out求和_平均值_
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] num = { 12, 25, 33, 42, 36 };
            Max(num, out int sum, out double average, out int max, out int min);
            Console.WriteLine(sum+" "+average+" "+max+" "+min);
        }
        public static void Max(int[]num1,out int sum,out double average,out int max,out int min)
        {
            
            sum = 0;
            max = int.MinValue;
            min = int.MaxValue;
            average = 0;
            for (int i = 0; i < num1.Length; i++)
            {
                if (num1[i] > max)
                {
                    max = num1[i];
                }
                if (num1[i] < min)
                {
                    min = num1[i];
                }
                sum += num1[i];
            }
            average = sum / num1.Length;
        }
    }
}

3.函数的递归

using System;

namespace _010_函数的递归调用
{
    class Program
    {
       public static  int F(int n)
        {
            if (n == 0) return 2;
            if (n == 1) return 3;
            return F(n - 1) + F(n - 2);
           
        }
        static void Main(string[] args)
        {
            //f(n)=f(n-1)+f(n-2) f(0)=2 f(1)=3 ,用程序求得f(40)
            int res = F(40);
            Console.WriteLine(res);

        }
    }
}

public static int i = 0;
        static void Main(string[] args)
        {
            //递归：自己的方法调用自己的方法就是递归
            //int num=0;
            //SayStory();
            //用递归求100以内正数的和
            int num = 100;
            int sum = GetSum(100);// getSumDG(99)+100//getSumDG(98)+99+100
            sum = GetSum(99) + 100;
            sum = (GetSumDG(98) + 99) + 100;
            sum = (GetSumDG(97) + 98) + 99 + 100;
        }
        public static int GetSumDG(int i)
        {
            if (i == 1)
            {
                return 1;
            }

            return GetSumDG(i - 1) + i;
            // getSumDG(99)+100

            //getSumDG(98)+99+100
            // .....
            // GetSumDG(1)+2+...+100

        }
        public static int GetSum(int i)
        {
            int sum = 0;
            //for (i = 1; i <= 100; i++)
            //{
            //    sum += i;// 1+2+3+4+5+6....+99+100
            //}
            for (i = 100; i > 0; i--)
            {
                sum += i;//100+99+98+97+.....3+2+1
            }
            return sum;


        }
        public static void SayStory()
        {
            // int i = 0;
            if (i == 3)
            {
                return;
            }
            i++;
            Console.WriteLine("从前山里有个庙");
            Console.WriteLine("庙里有个老和尚跟一个小和尚");
            Console.WriteLine("老和尚给小和尚讲故事");
            Console.WriteLine("两个和尚没水喝");
            SayStory();


        }
        public static void SayStory(int i)
        {
            // int i = 0;
            if (i == 3)
            {
                return;
            }
            i += 1;
            Console.WriteLine("从前山里有个庙");
            Console.WriteLine("庙里有个老和尚跟一个小和尚");
            Console.WriteLine("老和尚给小和尚讲故事");
            Console.WriteLine("两个和尚没水喝");
            SayStory(i);
        }

using System;

namespace _012阶乘
{
    class Program
    {
        public static int JC(int n)
        {
            int res = 1;
            for (int i = 1; i <= n; i++)
            {
                res *= i;
            }
            return res;
        }
        static void Main(string[] args)
        {
            int jc = 1;
            int sum = 0;
            for (int i = 1; i <= 20; i++)
            {
                //求1+2!+3!+...+20!的和?
                //jc = jc * i;
                //sum += jc;
                sum += JC(i);
                
            }
            Console.WriteLine(sum);
        }
    }
}

1.

using System;

namespace _013_判断质数
{

    class Program
    {
        static void Main(string[] args)
        {
            // 要求：重复让用户输入一个数,判断该数是否是质数,输入q结束? 质数的判断用方法来实现bool IsPrime(int number)
            //输入2时有bug
            Console.WriteLine("请输入一个数（输入q退出）：");
            string str = Console.ReadLine();
            while (str != "q")
            {
                int num;
                while (!int.TryParse(str, out num))
                {
                    Console.WriteLine("你刚刚输入的不是一个数字，请重新输入（输入q，退出）：");
                    str = Console.ReadLine();
                    if (str == "q")
                    {
                        return;
                    }
                }
                //if(num == 1)
                //{
                //    Console.WriteLine("1不是质数");
                //    str = Console.ReadLine();
                //}
                bool result = IsPrime(num); //调用方法，判断是否为质数
                if (result)
                {
                    Console.WriteLine("{0}是质数", num);
                    str = Console.ReadLine();
                }
                if (result == false)
                {
                Console.WriteLine(num+"不是质数，请再输入一个数（q退出）");
                str = Console.ReadLine();

                }

            }


            Console.ReadKey();
        }

        public static bool IsPrime(int number)
        {
            bool result = false;

            for (int i = number - 1; i >1; i--)
            {
                if (number % i == 0) //如果能整除，则不是质数
                {
                    result = false;
                }
                if (number % i != 0)//是质素
                {
                    result = true;
                }
            }

            return result; //注意return的位置，如果写在for 循环内，发生错误，不能----“返值”

        }

    }
}

 static void Main(string[] args)
        {
            //重复让用户输入一个数,判断该数是否是质数,输入q结束?质数的判断用方法来实现bool IsPrime(int number)

            Console.WriteLine("请输入一个数字");
            string num = Console.ReadLine();
            GetZS(num);

        }
        public static void GetZS(string num) {

            while (true)
            {
                if (num == "q")
                {
                    break;
                }
                else
                {
                    IsPrime(GetNum(num));
                    num = Console.ReadLine();
                }

            }
        
        }
        public static int GetNum(string s)
        {
            while (true)
            {
                try
                {
                    int num = int.Parse(s);
                    return num;
                }
                catch
                {
                    Console.WriteLine("您输入有误，重新输入");
                    s = Console.ReadLine();

                }
            }


        }
        public static bool IsPrime(int num)
        {

            for (int i = 2; i <= num; i++)
            {
                if (num % i == 0 && num != i)
                {
                    Console.WriteLine("不是质数");
                    return false;
                  
                }
                else if (i == num)
                {
                    Console.WriteLine("是质数");
                    return true;
                }


            }
            return false;

        }

4.try.pase

using System;

namespace _017_try.pase
{
    class Program
    {
        static void Main(string[] args)
        {
          

            Console.WriteLine("请输入账号和密码");
            string name = Console.ReadLine();
            string password = Console.ReadLine();
            string massage;
            Name(name, password, out massage);
            Console.WriteLine(massage);
        }
        public static void Name(string name ,string password,out string massage)
        {
            massage = "";
            if (name == "admin" && password == "123456")
            {
                massage = "登录成功";
            }
            else
            {
                massage = "失败";
            }
        } 
        public static bool TryPase(string num,out int number)
        {
            number = 0;
            try
            {
                number = int.Parse(num);
                return true;
            }
            catch 
            {
                return false;
               
            }
        }
    }
}

5.面向对象

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

namespace _001_面向对象
{
    public class People
    {
        private string _name;
        public int _age;
        private char _sex;

        public char Sex
        {
            get { return _sex; }
            set
            {
                if (value != '男' && value != '女')
                {
                    value = '中';
                }
                _sex = value;
            }
        }
        public int Age
        {
            get
            {
                return _age;
            }
            set
            {
                if (value < 0 || value > 100)
                {
                    value = 0;
                }
                _age = value;
            }

        }
        public People(string name, int age) //构造函数，用于构造对象，如果类里面没有该无参构造函数，就系统默认调用一个无参的构造函数
        {
            //只要自己定义了一个构造函数，系统中默认的无参的构造函数就不存在了
            this.Name = name;//构造函数结构，public+类名+（）{} ，无返回值，void都没有
            this.Age = age;
        }
        public People() //构造函数是可以构成重载的
        {


        }
        public People(string name, int age, char sex)
        {
            this.Name = name;
            this.Age = age;
            this.Sex = sex;
        }

        public string Name
        {
            get { return _name; }
            set
            {
                if (value != "张三")
                {
                    value = "张三";
                }
                _name = value;
            }
        }
        public void CHLSS()
        {
            Console.WriteLine("{0}今年{1}岁了，性别{2},可以吃喝拉撒睡", this.Name, this.Age, this.Sex);
        }
    }
}

using System;

namespace _001_面向对象
{
    class Program
    {
        static void Main(string[] args)
        {
            People zsPeople = new People("李四", 20, '女');//类的实例化,new的作用开辟一个内存
            zsPeople.CHLSS();
            //类：这个类具有所有人类的特性，是把对象中的共同点进行提炼抽象化出来的类
            //对象：类实例化出来的，他是实际存在的，
            //public 公共的，外界可以调用
            //private 私有的，只能在本类中调用
            //zsPeople.Age = 15;
            //zsPeople.Sex = 'a';//对象的初始化
            //zsPeople.Name = "张";
            //zsPeople.CHLSS();
            Console.ReadKey();


        }
    }
} 

using System;

namespace _002集合
{
    class Program
    {
        static void Main(string[] args)
        {
            _Student hwPeople = new _Student("黄伟",96,88,64);
            hwPeople.ZWJS();
        }
    }
}

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

namespace _002集合
{
   public class _Student
    {
        public string _name;
        public char _sex;
        public int _age;
        public double _chinese;
        public double _math;
        public double _English;
        public double _Sum;
        public double _Average;

        public string Name
        {
            get
            {
                return _name;
            }
            set
            {
                _name = value;
            }
        }

        public char Sex
        {
            get
            {
                return _sex;
            }
            set
            {
                _sex = value;
            }
        }

        public int Age
        {
            get
            {
                return _age;
            }
            set
            {
                _age = value;
            }
        }

        public double Chinese
        {
            get
            {
                return _chinese;
            }
            set
            {
                _chinese = value;
            }
        }

        public double Math
        {
            get
            {
                return _math;
            }
            set
            {
                _math = value;
            }
        }

        public double English
        {
            get
            {
                return _English;
            }
            set
            {
                _English = value;
            }
        }

        public _Student(string name, double chinese, double math, double English)
        {
            this.Name = name;
            this.Chinese = chinese;
            this.Math = math;
            this.English = English;
        }
        
       
        public void ZWJS()
        {
            //Console.WriteLine("我叫{0}，今年{1}岁了，是{2}同学",this.Name,this.Age,this.Sex);
            double sum = 0;
            sum = this.English + this.Chinese + this.Math;
            double average = sum / 3;
            Console.WriteLine("我叫"+this.Name+ "这次总成绩是"+sum +"分，平均成绩是"+average);
        }
    }
} 

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

namespace _004_Ticket
{
    class Ticket
    {
        private int _distance;
        private double _price;

        private int Distance
        {
            get
            {
                return _distance;
            }
            set
            {
                if (value < 0)
                {
                    value= 0;
                }
                _distance = value;
            }
        }

        private double Price
        {
            get
            {
                return _price;
            }
            set
            {
                if (_distance > 101 && _distance < 200)
                {
                    value = value * 0.95;
                }
                if (_distance > 201 && _distance < 300)
                {
                    value = value * 0.9;
                }
                if (_distance > 300)
                {
                    value = value * 0.8;
                }


                _price = value;

                
            }
        }

        public Ticket(int distance,double price)
        {
            this.Distance = distance;
            this.Price = price;
        }

        public void JiaGe()
        {
            double jiage = 0;
            jiage = this.Price * this.Distance;
            Console.WriteLine(this.Distance+"公里"+jiage+"块钱");
        }
    }
}

using System;

namespace _004_Ticket
{
    class Program
    {
        static void Main(string[] args)
        {
            Ticket Get = new Ticket(90, 1);
            Get.JiaGe();
        }
    }
} 

6.

字符串

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

namespace _001_字符串
{
    class Program
    {
        static void Main(string[] args)
        {
            //string a = "huangwei";//我们使用string类型去存储字符串类型，字符串 需要使用双引号引起来
            //int length = a.Length;//通过length获取字符串长度，空格也属于字符
            //Console.WriteLine(length);
            //char s = a[0];//通过索引取得字符串中的字符
            //a = "http://" + a;//字符串的链接
            //bool b =a.Contain("w");//是否包含某个字符串
            //a=a.Insert(2,"514")//在指定位置插入字符串
            //for (int i = 0; i < a.Length; i++)
            //{
            //    Console.WriteLine(a[i]);//字符串的遍历
            //}
            //int res = a.CompareTo("hiangwei");
            //当两个字符串相等的时候，返回0，    
            // int index=  s.IndexOf('4');//查找当前元素所在索引，查找到第一个索引为其元素时停止
            //string str = a.Replace('h', 'H');//把指定的字符换成指定的字符
            //string str1 = a.Replace("huang","HUANG");//把指定字符串换成指定字符串
            //Console.WriteLine(str);
            //Console.WriteLine(str1);
            // string s = "www.baidu.com";
            //s=  s.Substring(index+1);//截取字符串包含当前索引的值
            // string s=  str.Clone().ToString();//克隆一个字符串变成object类型，我们强转成字符串，

            //（一切类型+.ToString()都会变成字符串）
            // Console.WriteLine(index);
            //int num = 123;
            //string s = num.ToString();

            //bool b=  string.IsNullOrEmpty(str);
            //bool bb= str.EndsWith('a');
            //bool bbb=str.StartsWith('a');

            //string[] array = s.Split('.');//以指定字符拆分成字符串数组
            // foreach (var item in array)
            // {
            //     Console.WriteLine(item);
            // }
            // s = string.Join("|", array);
            // Console.WriteLine(s);//把字符串数组中添加一个字符把其串联成一个新的字符串
            //string str = s.Substring(4, 5);//按指定索引位置开始取子字符串，后面不写，表示 去到最后
            //Console.WriteLine(str);

            //string s = "huangWEI";//所有字符转换为小写形式
            //string str = s.ToLower();
            //String str1 = s.ToUpper();//所有字符转换为大写
            //Console.WriteLine(str);

            //string s = "www.baidu.com";
            //string str = s.Trim();//删除两端的空白
            //Console.WriteLine(str);
            //int index = s.IndexOf("baidu");//判断当前字符串是否包含一个子字符串，如果不包含，则返回-1，如果包含，会返回第一个字符的索引
            //Console.WriteLine(index);
            //string s = "huangwwei";
            //s = s.Remove(5, 2);
            //Console.WriteLine(s);//从当前索引删除指定长度字符串
        }
    }
}

stringBulider:
  string s = "123";//每次赋值都会重新申请内存
            StringBuilder ss = new StringBuilder();//效率很快，只需要一片内存
            ss.Append("123");
            Stopwatch sw = new Stopwatch();
            sw.Start();
            for (int i = 0; i < 100000; i++)
            {
                 s += i;//00:00:10.3843778
        // ss.Append(i);//00:00:00.0111952
            }
            sw.Stop();
            // Console.WriteLine(ss.ToString());
            Console.WriteLine(s);
            Console.WriteLine(sw.Elapsed);


//bool b=  string.IsNullOrEmpty(str);
            //bool bb= str.EndsWith('a');
            //bool bbb=str.StartsWith('a');

7.继承

 class Program
    {
        static void Main(string[] args)
        {
            // Teacher t = new Teacher();//继承的好处：把具有相同特征的类的重复的方法和属性提炼到一个类里面，省略冗余代码
            Student s = new Student("张三",  30);//子类继承于父类，拥有父类的一切特征，创建对象时要走一遍父类的构造函数,子类不能继承父类的构造函数
            //s.Name = "张三";
            //s.Age = 20;
            //s.Sex = '男';
            s.Study1();
            //  Driver d = new Driver();
            // Person p = new Person();
            // p.Say();
        }
    }
    public class Person
    {
        private string name;
        private int age;
        private char sex;
        public Person(string name, int age, char sex)
        {
            Name = name;
            Age = age;
            Sex = sex;

        }
        public string Name { get => name; set => name = value; }
        public int Age { get => age; set => age = value; }
        public char Sex { get => sex; set => sex = value; }
        public void Say()
        {

            Console.WriteLine("我是人类");

        }

    }
    public class Teacher : Person
    {
        //private string name;
        //private int age;
        //private char sex;

        //public string Name { get => name; set => name = value; }
        //public int Age { get => age; set => age = value; }
        //public char Sex { get => sex; set => sex = value; }
        public Teacher(string name, int age, char sex) : base(name, age, sex)
        {

        }
        public void Teach()
        {

            Console.WriteLine("老师叫{0}，今年{1}岁了，性别{2}，他会教书", Name, Age, Sex);

        }
    }
    public class Student : Person
    {
        private int _id;
        //private string name;
        //private int age;
        //private char sex;

        //public string Name { get => name; set => name = value; }
        //public int Age { get => age; set => age = value; }
        //public char Sex { get => sex; set => sex = value; }
        public Student(string name, int age, char sex, int id) : base(name, age, sex)
        {
            //Name = name;
            //Age = age;
            //Sex = sex;
            this.Id = id;

        }
        public Student(string name,int id):base(name,0,'a')
        {
            this.Id = id;
        }
        public void Study1() {
            Console.WriteLine("我叫{0}。我的学号是{1}",Name,this.Id);

        }
        public int Id { get => _id; set => _id = value; }

        public void Study()
        {

            Console.WriteLine("学生叫{0}，今年{1}岁了，性别{2},学号{3}，他会学习", Name, Age, Sex, this.Id);

        }
    }
    public class Driver : Person
    {

        //private string name;
        //private int age;
        //private char sex;

        //public string Name { get => name; set => name = value; }
        //public int Age { get => age; set => age = value; }
        //public char Sex { get => sex; set => sex = value; }
        public Driver(string name, int age, char sex) : base(name, age, sex)
        {
            //Name = name;
            //Age = age;
            //Sex = sex;

        }
        public void Drive()
        {

            Console.WriteLine("司机叫{0}，今年{1}岁了，性别{2}，他会开车", Name, Age, Sex);

        }


    }

例子：

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

namespace _004继承习题
{
    class Program
    {
        static void Main(string[] args)
        {
            Programmer s = new Programmer("黄伟",22,10);
            s.Jc1();

            Reproter a = new Reproter("谭红", 52, "吃翔");
            a.JS();
        }
       
    }
    public class Person
    {
        private string name;
        private int age;

        public Person(string name,int age)
        {
            Name = name;
            Age = age;
        }

        public string Name { get => name; set => name = value; }
        public int Age { get => age; set => age = value; }

    }

    public class Reproter : Person
    {
        private string aihao;
        public Reproter(string name, int age, string aihao):base(name,age)
        {
            this.Aihao = aihao;
        }

        public void JS()
        {
            Console.WriteLine("大家好，我叫{0}，今年{1}岁，我的爱好是{2}", Name, Age, this.aihao);
        }
        public string Aihao { get => aihao; set => aihao = value; }
    }

    public class Programmer : Person
    {
        private int time;
        public Programmer(string name, int age, int time):base(name,age)
        {
            this.time = time;
        }
        public void Jc1()
        {
            Console.WriteLine("大家好，我叫{0}，今年{1}，我已经工作{2}年了", Name, Age, this.time);
        }
        public int Time { get => time; set => time = value; }
    }
}

继承随机生成：

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

namespace _005继承随机
{
    class Program
    {
        static void Main(string[] args)
        {
            int num = 0;
            Person[] ps = { new Student(), new Teacher(), new Beauty(), new Animal(), new Ren() };
                Random random = new Random();//注意！！！别放里面
            for (int i = 0; i < 10; i++)
            {
                num = random.Next(0, 5);
                //for (int j = 0; j < 10; j++)
                //{
                    if (ps[num] is Student)
                    {
                        Student psStudent = (Student)ps[num];
                        psStudent.GetStudent();
                    continue;
                    }
                    else if (ps[num] is Teacher)
                    {
                        Teacher psTeacher = (Teacher)ps[num];
                        psTeacher.GetTeacher();
                    continue;
                }
                    else if (ps[num] is Beauty)
                    {
                        Beauty psBeauty = (Beauty)ps[num];
                        psBeauty.GetBeauty();
                    continue;
                }
                    else if (ps[num] is Animal)
                    {
                        Animal psAnimal = (Animal)ps[num];
                        psAnimal.GetAnimal();
                    continue;
                }
                    else if (ps[num] is Ren)
                    {
                        Ren psRen = (Ren)ps[num];
                        psRen.GetPerson();
                    continue;
                }
                    else
                    {
                        Console.WriteLine("转不了");
                        break;
                    }
                //}
            }
                    

        }
    }
    public class Person
    {
        private string name;
        private int age;

        public string Name { get => name; set => name = value; }
        public int Age { get => age; set => age = value; }


    }
    public class Ren : Person
    {
        public void GetPerson()
        {
            Console.WriteLine("我是人类");
        }
    }
    public class Man : Person
    {
        public void GetMan()
        {
            Console.WriteLine("我是帅哥");
        }
    }

    public class Teacher : Person
    {
        public void GetTeacher()
        {
            Console.WriteLine("我是老师");
        }
    }

    public class Student : Person
    {
        public void GetStudent()
        {
            Console.WriteLine("我是学生");
        }
    }

    public class Animal : Person
    {
        public void GetAnimal()
        {
            Console.WriteLine("我是野兽");
        }
    }

    public class Beauty : Person
    {
        public void GetBeauty()
        {
            Console.WriteLine("我是美女");
        }
    }
}

7.里式转换

里氏转换：

 class Program
    {
        static void Main(string[] args)
        {
            //里氏转换：1.子类可以代替父类（给父类赋值）2.如果父类里装的是子类对象，那么我们就可以把该父类强转为子类
            //Person p = new Person();
            //p.SayPerson();
            //Student s = new Student();
            //s.SayStudent();

            Person p = new Student();
            p.SayPerson();//父类对象被子类赋值，只能调用到父类的方法
            Person p1 = new Teacher();
            p1.SayPerson();
            //if (p is Teacher) //is 判断转换类型是否为该类型
            //{

            //    Teacher s = (Teacher)p;
            //    s.SayTeacher();
            //    s.SayPerson();
            //}
            //else
            //{
            //    Console.WriteLine("转不了");
            //}
          Student ss=  p as Student; // as 强制转换为子类对象，如果不是该类型就报错
            ss.SayStudent();


        }
    }
    public class Person
    {

        string _name;
        private int _age;
        private char _sxe;
        
// string _name;//private私有的只能在本类调用,如果不用private 默认私有,子类不能调用父类的私有字段，私有方法
        // public string age;//共有的
        // protected char sex;//受保护的，只能在继承关系里使用，用于保护父类的字段方法，只能子类和本类调用（基类和派生类）
        //internal 只能用于修饰类，保护程序集的，只能在当前程序集调用
        //protected internal 程序集里的父子关系的调用

        public string Name { get => _name; set => _name = value; }
        public int Age { get => _age; set => _age = value; }
        public char Sxe { get => _sxe; set => _sxe = value; }
        public void SayPerson()
        {

            Console.WriteLine("我是人类");
        }
    }
    public class Student : Person
    {

        public void SayStudent()
        {

            Console.WriteLine("我是学生");
        }

    }
    public class Teacher : Person
    {

        public void SayTeacher()
        {
            Console.WriteLine("我是老师");
        }
    }