C ＃ basic knowledge (2)

Write it at the front

Judge 、 loop 、 Construction method 、 Method overloading, etc , and java The language is basically similar to .
C# Medium goto、 Destructor 、readonly Read only objects 、partial Part of the class is java Not found in .

1、 Judgment and circulation

break Used to interrupt the loop
continue Skip this cycle

if…else

class Program
{
    
    static void Main(string[] args)
    {
    
        Console.WriteLine(" Please enter an integer ：");
        // Convert the value entered from the console to int type 
        int num = int.Parse(Console.ReadLine());
        if (num % 2 == 0)
        {
    
            Console.WriteLine(num+" yes 2 Multiple ！");
        }else if(num % 3 == 0){
    
    		Console.WriteLine(num+" yes 3 Multiple ！");
		}
        else
        {
    
            Console.WriteLine(num+" No 2 or 3 Multiple ！");
        }
    }
}

switch…case

class Program
{
    
    static void Main(string[] args)
    {
    
        Console.WriteLine(" Please enter the grade （0~100 The integer of ）");
        int points = int.Parse(Console.ReadLine());
        switch (points / 10)
        {
    
            case 10:
                Console.WriteLine(" good ");
                break;
            case 9:
                Console.WriteLine(" pass ");
                break;
            default:
                Console.WriteLine(" fail, ");
                break;
        }
    }
}

If case 10 and case 9 The output content is the same , You can do something like this

class Program
{
    
    static void Main(string[] args)
    {
    
        Console.WriteLine(" Please enter the student's test score （0~100 The integer of ）");
        int points = int.Parse(Console.ReadLine());
        if(points < 0 || points > 100)
        {
    
            points = 0;
        }
        switch (points / 10)
        {
    
            case 10:
            case 9:
                Console.WriteLine(" good ");
                break;
            case 8:
                Console.WriteLine(" good ");
                break;
            case 7:
            case 6:
                Console.WriteLine(" pass ");
                break;
            default:
                Console.WriteLine(" fail, ");
                break;
        }
    }
}

for

class Program
{
    
    static void Main(string[] args)
    {
    
        // Set the variables for storing and 
        int sum = 0;
        for(int i = 1; i <= 10; i++)
        {
    
            Console.WriteLine(i);
            sum += i;
        }
        Console.WriteLine("1~10 And for ：" + sum);
    }  
}

while

class Program
{
    
    static void Main(string[] args)
    {
    
        int i = 1;
        int sum = 0;// Deposit 1~10 And 
        while (i <= 10)
        {
    
            sum = sum + i;
            Console.WriteLine(i);
            i++;
        }
        Console.WriteLine("1~10 And for ：" + sum);
    }  
}

do while

class Program
{
    
    static void Main(string[] args)
    {
    
        int i = 1;
        do
        {
    
            Console.WriteLine(i);
            i++;
        } while (i <= 10);
    }  
}

goto

class Program
{
    
    static void Main(string[] args)
    {
    
        int count = 1;
    login:
        Console.WriteLine(" Please enter a user name ");
        string username = Console.ReadLine();
        Console.WriteLine(" Please input a password ");
        string userpwd = Console.ReadLine();
        if (username == "aaa" && userpwd == "123")
        {
    
            Console.WriteLine(" Login successful ");
        }
        else
        {
    
            count++;
            if (count > 3)
            {
    
                Console.WriteLine(" Too many user name or password errors ！ sign out ！");
            }
            else
            {
    
                Console.WriteLine(" Wrong user name or password ");
                goto login;// return login Re enter the user name and password at the label 
            }
        }
    }
}

2、 class 、 object 、 Method

There can be multiple classes in a namespace

using System;

namespace code_1
{
    
    class Test
    {
    
    }
    class Test1
    {
    
    }
}

The concrete syntax form of class definition is as follows .

 Class access modifier      Modifier      Class name 
{
    
     Class members 
}

among ：

• Class access modifier ： Used to set access restrictions on classes , Include public、internal Or don't write , use internal Or not writing means that the class can only be accessed in the current project ;public Represents that the class can be accessed in any project .
• Modifier ： A modifier is a description of the characteristics of a class itself , Include abstract、sealed and static.abstract It means Abstract , Classes that use its modifier cannot be instantiated ;sealed The decorated class is the sealed class , You can't Be inherited ;static The decorated class is a static class , Can't be instantiated .
• Class name ： The class name is used to describe the function of a class , Therefore, it is better to have practical significance when defining class names , This makes it easy for users to understand the content described in the class . Class names must be unique in the same namespace .
  Class members ： Elements that can be defined in a class , It mainly includes fields 、 attribute 、 Method .

After the field is defined in the class , When the class loads , The field will be automatically assigned .

namespace code_1
{
    
    class Test
    {
    
        private int id;                         // Define private integer fields  id, Automatic assignment 0
        public readonly string name;            // Define public read-only string type fields  name, Automatically assign a null value 
        internal static int age;                // Define internal static integer fields  age, Automatic assignment 0
        private const string major = " Computer ";  // Define private string type constants  major, There is an initial value 
    }
}

2.1、readonly Read only objects

stay C# There are several key points in using read-only fields ：
　　（1） Read only fields can be assigned together in the definition or in the structural method of the class ;
　　（2） In addition to the structural approach , The value of the read-only field cannot be modified elsewhere ;
　　（3） The characteristics of read-only fields can only be get accessor , Can not have set, It's obvious ;

2.2、const Constant

const Constants can only be initialized together with declarations （ assignment ）.
readonly Fields can be initialized in declaration or structure functions .

2.3、internal Internal object

internal（ Inside ）： The limitation is that it can only be accessed in the same assembly , Can cross class

2.4、static Static objects

There is only one static object , All static members are created in the “ Static storage area ” Inside , Once created, until the program exits , To be recycled .

2.4、get、set accessor

namespace code_1
{
    
    class Book
    {
    
        private int id;
        private string name;
        private double price;
        private String desc;

        public int Id
        {
    
            get
            {
    
                return id;
            }
            set
            {
    
                id = value;
            }
        }

		//  It can also be abbreviated in this way 
		public string Name{
    get; set;}

		//  Read only form 
		public double price{
    get;}=10.5;

		//  If you do not allow other classes to access property values 
		public String desc{
    private get; set;}
    }
}

2.5、 Calling class members

class Book
{
    
    public int Id {
     get; set; }
    public string Name {
     get; set; }
    public double Price {
     get; set; }
    public void PrintMsg()
    {
    
        Console.WriteLine(" Book number ：" + Id);
        Console.WriteLine(" The name of the book ：" + Name);
        Console.WriteLine(" The book price ：" + Price);
    }
}

class Program
{
    
    static void Main(string[] args)
    {
    
        Book book = new Book();
        // Assign values to properties 
        book.Id = 1;
        book.Name = " Fundamentals of computer ";
        book.Price = 34.5;
        book.PrintMsg();
    }
}

take Book The properties and methods in the class are changed to static

class Book
{
    
    public static int Id {
     get; set; }
    public static string Name {
     get; set; }
    public static double Price {
     get; set; }
    public static void SetBook(int id, string name, double price)
    {
    
        Id = id;
        Name = name;
        Price = price;
    }
    public static void PrintMsg()
    {
    
        Console.WriteLine(" Book number ：" + Id);
        Console.WriteLine(" The name of the book ：" + Name);
        Console.WriteLine(" The book price ：" + Price);
    }
}

class Program
{
    
    static void Main(string[] args)
    {
    
        Book.SetBook(1, " Fundamentals of computer ", 34.5);
        Book.PrintMsg();
    }
}

2.6、 Constructors

Constructors are used to execute when creating objects of classes

class User
{
    
	//  The constructor with the same name as the class 
    public User(string name, string password, string tel)
    {
    
        this.Name = name;
        this.Password = password;
        this.Tel = tel;
    }
    public string Name {
     get; set; }
    public string Password {
     get; set; }
    public string Tel {
     get; set; }
    public void PrintMsg()
    {
    
        Console.WriteLine(" user name ：" + this.Name);
        Console.WriteLine(" The secret   code ：" + this.Password);
        Console.WriteLine(" cell-phone number ：" + this.Tel);
    }
}


//  Call the instance 
class Program
{
    
    static void Main(string[] args)
    {
    
        User user = new User(" Xiao Ming ","123456","13131351111");
        user.PrintMsg();
    }
}

2.7、 Destructor

Deconstruction method in garbage collection 、 Use when releasing resources . The destructor method is called after the class operation is completed .

// ~ Class name , It's a deconstruction method 
~User()
{
    
    Console.WriteLine(" The destruct method is called ");
}

2.8、lambda expression

class LambdaClass
{
    
    public static int Add1(int a, int b) => a + b;
public static void Add2(int a, int b) => Console.WriteLine(a + b);
}

2.9、 Nested class

class OuterClass
{
    
    public class InnerClass
    {
    
        public string CardId {
     get; set; }
        public string Password {
     get; set; }
        public void PrintMsg()
        {
    
            Console.WriteLine(" The card number is ：" + CardId);
            Console.WriteLine(" The password for ：" + Password);
        }
    }
}


//  Calls to nested classes 
class Program
{
    
    static void Main(string[] args)
    {
    
        OuterClass.InnerClass outInner = new OuterClass.InnerClass();
        outInner.CardId = "622211100";
        outInner.Password = "123456";
        outInner.PrintMsg();
    }
}

2.10、partial Part of class

A class can consist of multiple parts , as follows

public partial class Course
{
    
    public int Id {
     get; set; }
    public string Name {
     get; set; }
    public double Points {
     get; set; }
}
public partial class Course
{
    
    public void PrintCoures()
    {
    
        Console.WriteLine(" Course number ：" + Id);
        Console.WriteLine(" Course name ：" + Name);
        Console.WriteLine(" course credit ：" + Points);
    }
}

partial It can also be used to modify methods ,
Some methods should be noted as follows 3 spot ：

• Some methods must be private , And can't use virtual、abstract、override、new、sealed、extern Equal modifier .
• Some methods cannot have a return value .
• Cannot be used in some methods out Parameters of type .

public partial class Course
{
    
    public int Id {
     get; set; }
    public string Name {
     get; set; }
    public double Points {
     get; set; }
    partial void PrintCourse();
    // call PrintCourse Method 
    public void PrintMsg()
    {
    
        PrintCourse();
    }
}
public partial class Course
{
    
    public void PrintCoures()
    {
    
        Console.WriteLine(" Course number ：" + Id);
        Console.WriteLine(" Course name ：" + Name);
        Console.WriteLine(" course credit ：" + Points);
    }
}