从矩形得出正方形是否违反了Liskov的替代原则？ [英] Is deriving square from rectangle a violation of Liskov's Substitution Principle?

问题描述

我是设计和学习设计原则的新手。

它表示从矩形获取矩形是违反Liskov替代原则的典型例子。

如果是这样，应该是正确的设计？

解决方案

我相信推理是这样的：

假设你有一个接受矩形并调整其宽度的方法：

  public void SetWidth（Rectangle rect，int width）
 {
 rect.Width = width; 
} 
  

应该是完全合理的，给定一个矩形，假设这个测试将通过：

  Rectangle rect = new Rectangle（50，20）; // width，height 
 
 SetWidth（rect，100）; 
 
 Assert.AreEqual（20，rect.Height）; 
  

...因为更改矩形的宽度不会影响其高度。

但是，假设您已经从Rectangle派生了一个新的Square类。根据定义，一个正方形的高度和宽度总是相等的。让我们再试一次：

  Rectangle rect = new Square（20）; // width和height 
 
 SetWidth（rect，100）; 
 
 Assert.AreEqual（20，rect.Height）; 
  

该测试将失败，因为将square的宽度设置为100也会更改其高度。 >

因此，通过从Rectangle派生Square来违反Liskov的替代原则。

is-a规则是有意义的真实世界（一个正方形绝对是一种矩形），但并不总是在软件设计领域。

编辑 / p>

为了回答你的问题，正确的设计应该是Rectangle和Square来自一个常见的Polygon或Shape类，它不执行任何关于宽度或高度。

I am new to design and learning the design principles.

It says deriving square from rectangle is a classic example of violation of Liskov's Substitution Principle.

If that's the case, what should be the correct design?

解决方案

I believe the reasoning is something like this:

Let's say you have a method that accepts a rectangle and adjusts its width:

public void SetWidth(Rectangle rect, int width)
{
    rect.Width = width;
}

It should be perfectly reasonable, given what a rectangle is, to assume that this test would pass:

Rectangle rect = new Rectangle(50, 20); // width, height

SetWidth(rect, 100);

Assert.AreEqual(20, rect.Height);

... because changing a rectangle's width does not affect its height.

However, let's say you've derived a new Square class from Rectangle. By definition, a square has height and width always equal. Let's try that test again:

Rectangle rect = new Square(20); // both width and height

SetWidth(rect, 100);

Assert.AreEqual(20, rect.Height);

That test will fail, because setting a square's width to 100 will also change its height.

Thus, Liskov's substitution principle is violated by deriving Square from Rectangle.

The "is-a" rule makes sense in the "real world" (a square is definitely a kind of rectangle), but not always in the world of software design.

Edit

To answer your question, the correct design should probably be that both Rectangle and Square derive from a common "Polygon" or "Shape" class, which does not enforce any rules regarding width or height.

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