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物件導向的多型機制，是指當兩個以上的類別繼承同一種父類別時，我們可以用父類別型態容納子類別的物件，真正進行函數呼叫時會呼叫到子類別的函數，此種特性稱之為多型。
以下是我們用 C# 實作的一個多型範例，在範例中，我們宣告了一個形狀類別，該類別具有一個 area() 函數可以計算該形狀的面積，然後我們又宣告了兩個子類別 Rectangle (矩形) 與 Circle (圓形)。我們將兩者放入到 shapes 陣列中，以便展示多型技巧，用父類別容器呼叫子類別的實體。
範例：形狀、矩形與圓形
using System;
class Shape
{
public virtual double area() { return 0.0; }
public static void Main(string[] args)
{
Rectangle r = new Rectangle(5.0, 8.0);
Console.WriteLine("r.area() = " + r.area());
Circle c = new Circle(3.0);
Console.WriteLine("c.area() = " + c.area());
Shape[] shapes = { r, c };
for (int i = 0; i < shapes.Length; i++)
Console.WriteLine("shapes[" + i + "].area()=" + shapes[i].area());
}
}
class Rectangle : Shape
{
double width, height;
public Rectangle(double w, double h)
{
width = w;
height = h;
}
public override double area()
{
return width * height;
}
}
class Circle : Shape
{
double r;
public Circle(double r)
{
this.r = r;
}
public override double area()
{
return 3.1416 * r * r;
}
}
執行結果
r.area() = 40
c.area() = 28.2744
shapes[0].area()=40
shapes[1].area()=28.2744
範例：使用抽象父型態
using System;
class ShapeTest
{
public static void Main(string[] args)
{
Rectangle r = new Rectangle(5.0, 8.0);
Console.WriteLine("r.area() = " + r.area());
Circle c = new Circle(3.0);
Console.WriteLine("c.area() = " + c.area());
Shape[] shapes = { r, c };
for (int i = 0; i < shapes.Length; i++)
Console.WriteLine("shapes[" + i + "].area()=" + shapes[i].area());
}
}
abstract class Shape
{
public abstract double area();
}
class Rectangle : Shape
{
double width, height;
public Rectangle(double w, double h)
{
width = w;
height = h;
}
public override double area()
{
return width * height;
}
}
class Circle : Shape
{
double r;
public Circle(double r)
{
this.r = r;
}
public override double area()
{
return 3.1416 * r * r;
}
}
執行結果：
r.area() = 40
c.area() = 28.2744
shapes[0].area()=40
shapes[1].area()=28.2744
範例：使用介面
using System;
class ShapeTest
{
public static void Main(string[] args)
{
Rectangle r = new Rectangle(5.0, 8.0);
Console.WriteLine("r.area() = " + r.area());
Circle c = new Circle(3.0);
Console.WriteLine("c.area() = " + c.area());
Shape[] shapes = { r, c };
for (int i = 0; i < shapes.Length; i++)
Console.WriteLine("shapes[" + i + "].area()=" + shapes[i].area());
}
}
interface Shape
{
double area();
}
class Rectangle : Shape
{
double width, height;
public Rectangle(double w, double h)
{
width = w;
height = h;
}
public double area()
{
return width * height;
}
}
class Circle : Shape
{
double r;
public Circle(double r)
{
this.r = r;
}
public double area()
{
return 3.1416 * r * r;
}
}
執行結果：
r.area() = 40
c.area() = 28.2744
shapes[0].area()=40
shapes[1].area()=28.2744
範例：較完整複雜的版本
using System;
class Shape
{
public double thick = 0.0;
public virtual double area() { return 0.0; }
public double volume()
{
return area() * thick;
}
public override String ToString()
{
return "Shape: thick=" + thick+" area="+area()+" volume="+volume();
}
}
class Rectangle : Shape
{
double width, height;
public Rectangle(double w, double h)
{
width = w;
height = h;
thick = 2.0;
}
public override String ToString()
{
return base.ToString()+" Rectangle:width=" + width + " height=" + height;
}
public override double area()
{
return width * height;
}
}
class Circle : Shape
{
double r;
public Circle(double r)
{
this.r = r;
thick = 3.0;
}
public override String ToString()
{
return base.ToString() + " Circle:r=" + r + " thick=" + thick;
}
public override double area()
{
return 3.1416 * r * r;
}
}
class Test
{
public static void Main(string[] args)
{
Shape s = new Shape();
Console.WriteLine("s.area() = " + s.area());
Rectangle r = new Rectangle(5.0, 8.0);
Console.WriteLine("r.area() = " + r.area());
Console.WriteLine("r.volume() = " + r.volume());
Circle c = new Circle(2);
Console.WriteLine("c.area() = " + c.area());
Console.WriteLine("c.volume() = " + c.volume());
c.thick = 5;
Console.WriteLine("c.volume() = " + c.volume());
Shape[] array = new Shape[] { s, r, c, r };
foreach (Shape o in array)
{
// Console.WriteLine("o.area()=" + o.area() +" o.volume()="+o.volume());
Console.WriteLine(o.ToString());
}
}
}
文章轉載：陳鍾誠 (2010年10月19日)，(網頁標題) C# 與物件導向中的多型技術 (Polymorphism)，(網站標題) 免費電子書：C# 程式設計，2010年10月19日，取自 http://cs0.wikidot.com/polymorphism ，網頁修改第 3 版。
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