C# 语法练习(11): 类[三] - 构造函数、析构函数、base、this

构造函数与析构函数:

---

using System;

class MyClass
{
    private int FNum;
    public int Num { get { return FNum; } }

    /* 构造函数没有返回值, 无参的构造函数是默认的 */
    public MyClass()
    {
        this.FNum = 2009;
    }

    /* 可以有多个参数不同的构造函数 */
    public MyClass(int x)
    {
        this.FNum = x;
    }

    public MyClass(params int[] arr)
    {
        foreach (int i in arr) this.FNum += i;
    }

    /* 析构函数无参、无返回值、无访问修饰, 最多只能有一个 */
    ~MyClass()
    {
        //析构函数是自动调用的
    }
}

class Program
{
    static void Main()
    {
        MyClass obj1, obj2, obj3;

        obj1 = new MyClass();
        Console.WriteLine(obj1.Num); //2009

        obj2 = new MyClass(100);
        Console.WriteLine(obj2.Num); //100

        obj3 = new MyClass(1, 2, 3);
        Console.WriteLine(obj3.Num); //6

        Console.ReadKey();
    }
}

如果没有构造与析构函数, new 时将使用默认(或继承); 给一个私有的构造函数能阻止类被实例化:

---

using System;

class MyClass
{
    private MyClass() { }
    public static void Msg1() { Console.WriteLine("Msg1"); }
    public static void Msg2() { Console.WriteLine("Msg2"); }
}

class Program
{
    static void Main()
    {
        MyClass.Msg1(); //Msg1
        MyClass.Msg2(); //Msg2

        Console.ReadKey();
    }
}

如果一个类有了非默认的构造函数, 就不能再使用默认的构造函数:

---

using System;

class MyClass
{
    private int FNum;
    public int Num { get { return FNum; } }

    public MyClass(int x, int y)
    {
        this.FNum = x + y;
    }
}

class Program
{
    static void Main()
    {
        MyClass obj;

        obj = new MyClass(1, 2);
        Console.WriteLine(obj.Num); //3

        Console.ReadKey();
    }
}

静态构造函数:

静态构造函数既无访问修饰符、无参数;
在 new 或调用任何静态成员之前，将自动调用静态构造函数;
静态构造函数一般用于初始化静态数据;
静态构造函数会在第一次 new 或第一次使用静态成员前触发;
不能直接调用静态构造函数.

---

using System;

class MyClass
{
    public static int Num;
    public static void ShowNum() { Console.WriteLine(Num); }
    public void Msg() { Console.WriteLine("Msg"); }

    static MyClass() { Num = 123; }
}

class Program
{
    static void Main()
    {
        MyClass.ShowNum();            //123
        MyClass.Num = 2009;
        MyClass.ShowNum();            //2009

        MyClass obj1 = new MyClass();
        obj1.Msg();                   //Msg

        Console.ReadKey();
    }
}

自动调用父类的构造方法:

---

using System;

class Parent
{
    public Parent() { Console.WriteLine("Parent"); }
}

class Child1 : Parent
{
    public Child1() { Console.WriteLine("Child1"); }
    public Child1(int x) { Console.WriteLine(x); }
}

class Child2 : Child1
{
    public Child2() { Console.WriteLine("Child2"); }
    public Child2(int x, int y) { Console.WriteLine(x + y); }
}

class Program
{
    static void Main()
    {
        Parent p = new Parent();           // Parent
        Child1 c1 = new Child1();          // Parent / Child1
        Child2 c2 = new Child2();          // Parent / Child1 / Child2

        Child1 c11 = new Child1(999);      // Parent / 999
        Child2 c22 = new Child2(111, 222); // Parent / Child1 / 333
        
        Console.ReadKey();
    }
}

base:

---

using System;

class Parent
{
    public Parent() { Console.WriteLine("Parent"); }
    public Parent(int x, int y) { Console.WriteLine(x + y); }
    public Parent(string s1, string s2) { Console.WriteLine(s1 + s2); }
}

class Child1 : Parent
{
    public Child1() { Console.WriteLine("Child1"); }
}

class Child2 : Parent
{
    public Child2() : base() { Console.WriteLine("Child2"); }
}

class Child3 : Parent
{
    public Child3() : base(111,222) { Console.WriteLine("Child3"); }
}

class Child4 : Parent
{
    public Child4() : base("111", "222") { Console.WriteLine("Child4"); }
}

class Program
{
    static void Main()
    {
        Child1 c1 = new Child1(); // Parent / Child1
        Child2 c2 = new Child2(); // Parent / Child2
        Child3 c3 = new Child3(); // 333 / Child3
        Child4 c4 = new Child4(); // 111222 / Child4
        
        Console.ReadKey();
    }
}

this:

---

using System;

class MyClass
{
    private string fs = "ABC-";
    public MyClass() { Console.WriteLine("MyClass"); }
    public MyClass(string str) { Console.WriteLine(this.fs + str); }
    public MyClass(int num) : this() { Console.WriteLine(num); }
    public MyClass(int x, int y) : this("XYZ") { Console.WriteLine(x + y); }
}

class Program
{
    static void Main()
    {
        MyClass c1 = new MyClass();          // MyClass
        MyClass c2 = new MyClass("EFG");     // ABC-EFG
        MyClass c3 = new MyClass(123);       // MyClass / 123
        MyClass c4 = new MyClass(111, 222);  // ABC-XYZ / 333
        Console.ReadKey();
    }
}

构造函数、属性、base:

---

using System;

abstract class Parent
{
    private byte FID;

    public Parent(byte n)
    {
        FID = n;
    }

    public byte Id
    {
        get { return FID; }
        set { FID = value; }
    }
}

class Child : Parent
{
    public Child(byte MyID) : base(MyID) { }
}


class Program
{
    static void Main()
    {
        Child Rect = new Child(6);
        Console.WriteLine(Rect.Id); //6

        Rect.Id = 8;
        Console.WriteLine(Rect.Id); //8

        Console.ReadKey();
    }
}