类的继承(重写)

• 类成员的”横向扩展”：成员越来越多(上小节)
• 类成员的”纵向扩展”：行为改变，版本增高(即重写)
• 类成员的隐藏(工作中是需要避免的，不常用)
• 重写与隐藏的发生条件：函数成员(方法/属性)，可见(访问级别为public/protected)，签名一致
  • 重写的函数成员具体可以为以下：

• 示例：子类成员对父类成员的重写，父类成员用virtual修饰，子类成员用override修饰 ```csharp using System;

namespace OverrideExample { class Program { static void Main(string[] args) { Car car = new Car(); car.Run();//输出：Car is running!

        Vehicle v = new Vehicle();
        v.Run();//I'm running!
        Vehicle v1 = new Car();//Is a
        v1.Run();//输出：Car is running!
    }
}
class Vehicle
{
    public virtual void Run()
    {
        Console.WriteLine("I'm running!");
    }
}
class Car : Vehicle
{
    public override void Run()
    {
        Console.WriteLine("Car is running!");
    }
}

}

   - 如果父类成员不加virtual，子类成员不加override，则表示子类成员对父类成员的隐藏
   - 隐藏时Car类中有两个Run方法，一个this的Run，一个base的Run(继承父类的)
   - 隐藏时Vehicle v1 = new Car();   v1.Run()；输出I'm running!，而重写则Car类中只有一个版本的Run方法
- 示例：继续增加派生类，引申多态概念，父类类型的变量引用子类类型的实例
```csharp
using System;

namespace OverrideExample
{
    class Program
    {
        static void Main(string[] args)
        {
            Vehicle v = new RaceCar();//多态
            v.Run();//输出：RaceCar is running!
        }
    }

    class Vehicle
    {
        public virtual void Run()
        {
            Console.WriteLine("I'm running!");
        }
    }

    class Car : Vehicle
    {
        public override void Run()
        {
            Console.WriteLine("Car is running!");
        }
    }

    class RaceCar : Car
    {
        public override void Run()
        {
            Console.WriteLine("RaceCar is running!");
        }
    }
}

• 如果RaceCar的Run方法不写override修饰，则RaceCar类中有2个版本的Run方法，一个是自身的，一个是Car版本的
• 当Vehicle v = new RaceCar()时，变量v会顺着继承链向下找到Car时，Run不在向下重写，所以输出的结果为Car is running!，若想输出RaceCar is running!，则需改成RaceCar rc = new RaceCar()

多态

• 基于重写机制(virtual -> override)
• 函数成员的具体行为(版本)由对象决定
• 属性也是method member，所以也能重写
• 示例：属性的重写 ```csharp using System;

namespace OverrideExample { class Program { static void Main(string[] args) { Vehicle v = new Car(); v.Run();//输出：Car is running! Console.WriteLine(v.Speed);///输出：50 } }

class Vehicle
{
    private int _speed;

    public virtual int Speed
    {
        get { return _speed; }
        set { _speed = value; }
    }

    public virtual void Run()
    {
        Console.WriteLine("I'm running!");
        _speed = 100;
    }
}

class Car : Vehicle
{
    private int _rpm;

    public override int Speed
    {
        get { return _rpm / 100; }
        set { _rpm = value * 100; }
    }

    public override void Run()
    {
        Console.WriteLine("Car is running!");
        _rpm = 5000;
    }
}

} ```