多态性与铸造 [英] Polymorphism and casting

问题描述

我想尝试一些结构来理解多态性在C＃，所以我想出了以下情况：

I want to understand polymorphism in c# so by trying out several constructs I came up with the following case:

class Shape
{
    public virtual void Draw()
    {
        Console.WriteLine("Shape.Draw()");
    }
}

class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Circle.Draw()");
    }
}

据我所知，为了送抽奖（）消息给多个相关对象，因此它们可以根据它自己的执行行动我必须改变到的实例（本例中）的形状是定点，以

I understand that in order to send the Draw() message to several related objects, so they can act according to its own implementation I must change the instance to which (in this case) shape is 'pointing' to:

Shape shape = new Circle();
shape.Draw(); //OK; This prints: Circle.Draw()

但为什么，当我做到这一点：

But why, when I do this:

Circle circle = new Circle();
circle.Draw(); //OK; This prints: Circle.Draw()

Shape shape = circle as Shape; // or Shape shape = (Shape)circle;
shape.Draw();

它打印出：Circle.Draw（）

It prints: "Circle.Draw()"

为什么它会调用投后Circle.Draw（）代替Shape.Draw（）？什么是这种情况的原因何在呢？

Why it calls the Circle.Draw() instead Shape.Draw() after the cast? What is the reasoning for this?

推荐答案

铸造不会改变对象的运行时类型，什么实现每一个特定的虚方法比如有。

Casting does not change run-time type of object and what implementation of particular virtual method each instance have.

需要注意的是以下两个情况下，你必须为样本是相同的：

Note that following 2 cases you have as sample are identical:

Shape shape = new Circle();
shape.Draw(); //OK; This prints: Circle.Draw()

和

Circle circle = new Circle();
Shape shape = circle as Shape;
shape.Draw();

第一个是基本上与第二短版本。

The first one is essentially shorter version of the second.

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