程式設計筆記byChris

C#

// Define the base class
class Car
{
    public virtual void DescribeCar()
    {
        System.Console.WriteLine("Four wheels and an engine.");
    }
}

// Define the derived classes
class ConvertibleCar : Car
{
    public new virtual void DescribeCar()
    {
        base.DescribeCar();
        System.Console.WriteLine("A roof that opens up.");
    }
}

class Minivan : Car
{
    public override void DescribeCar()
    {
        base.DescribeCar();
        System.Console.WriteLine("Carries seven people.");
    }
}

public static void TestCars1()
{
    Car car1 = new Car();
    car1.DescribeCar();
    System.Console.WriteLine("----------");

    ConvertibleCar car2 = new ConvertibleCar();
    car2.DescribeCar();
    System.Console.WriteLine("----------");

    Minivan car3 = new Minivan();
    car3.DescribeCar();
    System.Console.WriteLine("----------");
}

執行結果

預期的輸出即如下：

Four wheels and an engine.

----------

Four wheels and an engine.

A roof that opens up.

----------

Four wheels and an engine.

Carries seven people.

----------

 但若宣告衍生自 Car 基底類別的物件陣列。這個陣列可以存放 Car、ConvertibleCar 和 Minivan 物件。陣列宣告如下：

 C#

public static void TestCars2()
{
    Car[] cars = new Car[3];
    cars[0] = new Car();
    cars[1] = new ConvertibleCar();
    cars[2] = new Minivan();
}

接著可以使用 foreach 迴圈探尋陣列中所包含的每一個 Car 物件，並呼叫 DescribeCar 方法，如下所示：

foreach (Car vehicle in cars)
{
    System.Console.WriteLine("Car object: " + vehicle.GetType());
    vehicle.DescribeCar();
    System.Console.WriteLine("----------");
}

這個迴圈的輸出如下：

Car object: YourApplication.Car

Four wheels and an engine.

----------

Car object: YourApplication.ConvertibleCar

Four wheels and an engine.

----------

Car object: YourApplication.Minivan

Four wheels and an engine.

Carries seven people.   

----------

請注意，ConvertibleCar 的描述和預期的不同。由於這個方法是使用 new 關鍵字所定義的，因此不會呼叫衍生類別方法，而是呼叫基底類別方法。Minivan 物件則是會正確呼叫覆寫方法，產生預期的結果。

文章轉自: http://msdn.microsoft.com/zh-tw/library/ms173153(VS.80).aspx