[C# Essentials] Derivation

Sealed Class

sealed 禁止任何类从其派生，例如string类

也可用于重载方法

Virtual

C#支持对属性和方法的重写，不支持字段和静态成员

用 virtual 关键字修饰基类中被重写的方法，override 修饰重写后的方法

使用override重写后，虚成员会默认调用 ”most derived implementation”


// Contact : Item
Item item;
Contact contact;
contact = new Contact();
item = contact;

item.virtualMethod(); // Call the implementation of "Contact" indeed

new V.S. override

override如上所述

new关键字修饰成员时，表明隐藏其基类成员。通过基类引用调用该成员时，仍调用基类的方法（静态类型only，dynamic修饰的动态类型反之）

派生类的同名方法默认为new


namespace TestUse;

public class Program {
    public class BaseClass {
        public void Display() {
            Console.WriteLine("BaseClass");
        }
    }

		// Compiler WARNING
		// Display' 上需要关键字 'new'，因为它会隐藏方法 'void TestUse.Program.BassClass.Display()'
    public class DerivedClass : BaseClass {
        public virtual void Display() {
            Console.WriteLine("DerivedClass");
        }
    }

    public class SubDerivedClass : DerivedClass {
        public override void Display() {
            Console.WriteLine("SubDerivedClass");
        }
    }

    public class SuperSubDerivedClass : SubDerivedClass {
        public new void Display() {
            Console.WriteLine("SuperSubDerivedClass");
        }
    }

    public static void Main() {
        SuperSubDerivedClass superSubDerivedClass = new SuperSubDerivedClass();
        SubDerivedClass subDerivedClass = superSubDerivedClass;
        DerivedClass derivedClass = superSubDerivedClass;
        BaseClass baseClass = superSubDerivedClass;
        
        superSubDerivedClass.Display();
        subDerivedClass.Display();
        derivedClass.Display();
        baseClass.Display();
    }

}

/*
SuperSubDerivedClass
SubDerivedClass
SubDerivedClass
BaseClass
*/

所有类的基类：System.Object

甚至字面值也支持System.Object 的方法


Console.WriteLine(42.ToString());

无论显式继承 object 与否，生成的CIL代码都相同

is

用于模式匹配

可匹配类型、常量、var


if (data is "") {
	// ...
}

// switch ...
else (GetObjectById(id) is var result) {
	// Equal to var result = GetObjectById(id)
}

C#8.0后：元组、按序、属性、递归模式匹配


// 元组
if ((args.Length, args[Actor] is (1, "show")){}

// 按序
// 对类的解构函数进行匹配
if (person is (string firstname, string lastname) {}


// 属性
// 同按序，但是可以指定属性名字
// 假定person的Deconstruct方法的out参数为
// firstname和lastname
person is {firstname: string firstname, lastnae: string lastname}

// 递归可以认为是上述模式匹配的组合

switch expression (C# 8.0)

下面这段代码即为switch expression的使用

其会将switch关键字前面的变量与大括号内的类型进行匹配，并返回一个可空元组 (int Year, int Month, int Day)?

is {} date 用于检查switch expression是否非空，是则将返回值命名为date，进行后续操作


public static string? CompositeFormatDate(object input, 
	string CompositeFormatString) {
	input switch
	{
    DateTime { Year: int year, Month: int month, Day: int day } 
	    => (year, month, day),
    DateTimeOffset { Year: int year, Month: int month, Day: int day } 
	    => (year, month, day),
    string dateText 
	    => DateTime.TryParse(dateText, out DateTime dateTime) ?
        (dateTime.Year, dateTime.Month, dateTime.Day) :
        default((int Year, int Month, int Day)?),
    _ => null
	} is {} date ? string.Format(CompositeFormatString, date.Year, 
		date.Month, date.Day) : null;
}