พหุรูปแบบและการเรียกแบบพลวัต

ภาพรวมพหุรูปแบบ

พื้นฐานการเลือกเมธอดแบบไดนามิก

using System;

public class Animal
{
  public virtual string Speak() { return "???"; }
}

public class Cat : Animal
{
  public override string Speak() { return "Meow"; }
}

public class Dog : Animal
{
  public override string Speak() { return "Woof"; }
}

public class Program
{
  public static void Main(string[] args)
  {
    Animal a1 = new Cat(); // base reference
    Animal a2 = new Dog();
    Console.WriteLine(a1.Speak()); // Meow
    Console.WriteLine(a2.Speak()); // Woof
  }
}

พหุรูปแบบในคอลเลกชัน

using System;
using System.Collections.Generic;

public class Shape
{
  public virtual double Area() { return 0.0; }
}

public class Rectangle : Shape
{
  private readonly int _w, _h;
  public Rectangle(int w, int h) { _w = w; _h = h; }
  public override double Area() { return _w * _h; }
}

public class Circle : Shape
{
  private readonly int _r;
  public Circle(int r) { _r = r; }
  public override double Area() { return 3.14159 * _r * _r; }
}

public class Program
{
  public static void Main(string[] args)
  {
    List<Shape> shapes = new List<Shape>();
    shapes.Add(new Rectangle(3, 4));
    shapes.Add(new Circle(2));

    foreach (Shape s in shapes)
    {
      Console.WriteLine("Area = " + s.Area()); // calls correct override
    }
  }
}

การซ่อนเทียบกับการแทนที่

using System;

public class Printer
{
  public virtual void Print() { Console.WriteLine("[Base]"); }
}

public class LoudPrinter : Printer
{
  public override void Print() { Console.WriteLine("[LOUD]"); }
}

public class HiddenPrinter : Printer
{
  // Hides the base method instead of overriding (BAD fit for polymorphism)
  public new void Print() { Console.WriteLine("[HIDDEN]"); }
}

public class Program
{
  public static void Main(string[] args)
  {
    Printer a = new LoudPrinter();
    Printer b = new HiddenPrinter();

    a.Print(); // [LOUD]  (override works)
    b.Print(); // [Base]  (hiding fails via base reference)
  }
}

การแปลงชนิดข้อมูลอย่างปลอดภัย (as และการตรวจสอบ)

using System;

public class Vehicle
{
  public virtual string Info() { return "Vehicle"; }
}

public class Car : Vehicle
{
  public bool HasAirConditioning;
  public override string Info() { return "Car"; }
}

public class Program
{
  public static void Main(string[] args)
  {
    Vehicle v = new Car();
    Console.WriteLine(v.Info()); // polymorphic call

    // Need a Car-only member? Cast carefully using 'as'
    Car c = v as Car;
    if (c != null)
    {
      c.HasAirConditioning = true;
      Console.WriteLine("AC set: " + c.HasAirConditioning);
    }
    else
    {
      Console.WriteLine("Not a Car");
    }
  }
}

เคล็ดลับเกี่ยวกับพหุรูปแบบ

ความหมายของการเลือกเมธอดแบบไดนามิก

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