C# 面向对象 - 继承与多态

11.1 继承基础

基本语法

// 基类
public class Animal
{
    public string Name { get; set; }
    
    public void Eat()
    {
        Console.WriteLine($"{Name} 正在吃东西");
    }
}

// 派生类
public class Dog : Animal
{
    public void Bark()
    {
        Console.WriteLine($"{Name} 汪汪叫");
    }
}

// 使用
Dog dog = new Dog { Name = "旺财" };
dog.Eat();   // 继承的方法
dog.Bark();  // 自己的方法

11.2 base 关键字

调用基类构造函数

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

public class Student : Person
{
    public string StudentId { get; set; }
    
    public Student(string name, int age, string studentId) 
        : base(name, age)  // 调用基类构造函数
    {
        StudentId = studentId;
    }
}

调用基类方法

public class Animal
{
    public virtual void MakeSound()
    {
        Console.WriteLine("动物发出声音");
    }
}

public class Cat : Animal
{
    public override void MakeSound()
    {
        base.MakeSound();  // 调用基类方法
        Console.WriteLine("喵喵叫");
    }
}

11.3 虚方法与重写

virtual 与 override

public class Shape
{
    public virtual double Area()
    {
        return 0;
    }
}

public class Rectangle : Shape
{
    public double Width { get; set; }
    public double Height { get; set; }
    
    public override double Area()
    {
        return Width * Height;
    }
}

public class Circle : Shape
{
    public double Radius { get; set; }
    
    public override double Area()
    {
        return Math.PI * Radius * Radius;
    }
}

多态示例

List<Shape> shapes = new()
{
    new Rectangle { Width = 5, Height = 3 },
    new Circle { Radius = 4 }
};

foreach (Shape shape in shapes)
{
    Console.WriteLine($"面积: {shape.Area():F2}");
}
// 输出:
// 面积: 15.00
// 面积: 50.27

11.4 隐藏基类方法

new 关键字

public class Base
{
    public void Show()
    {
        Console.WriteLine("Base.Show");
    }
}

public class Derived : Base
{
    public new void Show()  // 隐藏基类方法
    {
        Console.WriteLine("Derived.Show");
    }
}

Base b = new Derived();
b.Show();  // "Base.Show"（编译时类型决定）

Derived d = new Derived();
d.Show();  // "Derived.Show"

11.5 sealed 类与方法

密封类

// 不能被继承
public sealed class Singleton
{
    public static readonly Singleton Instance = new();
    private Singleton() { }
}

// public class Child : Singleton { }  // 错误

密封方法

public class Base
{
    public virtual void Process() { }
}

public class Derived : Base
{
    public sealed override void Process()
    {
        // 派生类不能再重写此方法
    }
}

11.6 多态

运行时多态

public class Employee
{
    public virtual decimal GetSalary() => 5000;
}

public class Manager : Employee
{
    public override decimal GetSalary() => 10000;
}

public class Developer : Employee
{
    public override decimal GetSalary() => 8000;
}

// 多态调用
List<Employee> employees = new()
{
    new Manager(),
    new Developer(),
    new Employee()
};

foreach (var emp in employees)
{
    Console.WriteLine($"薪资: {emp.GetSalary()}");
}
// 输出:
// 薪资: 10000
// 薪资: 8000
// 薪资: 5000

11.7 类型检查与转换

is 运算符

Employee emp = new Manager();

if (emp is Manager)
{
    Console.WriteLine("是经理");
}

// 模式匹配
if (emp is Manager m)
{
    Console.WriteLine($"经理薪资: {m.GetSalary()}");
}

as 运算符

Employee emp = new Manager();

Manager manager = emp as Manager;
if (manager != null)
{
    Console.WriteLine("转换成功");
}

// 失败返回 null
Developer dev = emp as Developer;  // null

强制转换

Employee emp = new Manager();

Manager manager = (Manager)emp;  // 成功
// Developer dev = (Developer)emp;  // 抛出异常

11.8 继承最佳实践

1. 组合优于继承

// 不推荐
class Car : Engine { }

// 推荐
class Car
{
    private Engine _engine;
    
    public Car(Engine engine)
    {
        _engine = engine;
    }
}

2. 使用抽象类定义模板

public abstract class ReportGenerator
{
    public void Generate()
    {
        LoadData();
        ProcessData();
        SaveReport();
    }
    
    protected abstract void LoadData();
    protected abstract void ProcessData();
    protected virtual void SaveReport() { }
}

3. 避免深层继承

// 不推荐：继承层级过深
class A { }
class B : A { }
class C : B { }
class D : C { }  // 难以维护

// 推荐：扁平化设计
interface IProcessable { }
class A : IProcessable { }
class B : IProcessable { }

11.9 本章小结

继承使用 : 语法，支持单继承
base 调用基类构造函数和方法
virtual 声明虚方法，override 重写
new 隐藏基类方法
sealed 防止继承或重写
多态允许基类引用调用派生类方法
is 检查类型，as 安全转换
组合通常优于继承

练习题

创建 Vehicle 基类，派生 Car 和 Bike
实现多态：不同形状计算面积
使用 base 调用基类构造函数

以下代码输出什么？

class A { public virtual void Show() => Console.WriteLine("A"); }
class B : A { public override void Show() => Console.WriteLine("B"); }
A obj = new B();
obj.Show();