利斯科夫替换原理

被重写的方法不应该保持为空 被重写的方法不应该抛出错误 基类或接口行为不应该因为派生类行为而进行修改(重做)。

这里有一个清单来确定你是否违反了利斯科夫法则。

如果你违反了以下项目之一->，你违反了里斯科夫。 如果你不违反任何->不能得出任何结论。

检查表:

No new exceptions should be thrown in derived class: If your base class threw ArgumentNullException then your sub classes were only allowed to throw exceptions of type ArgumentNullException or any exceptions derived from ArgumentNullException. Throwing IndexOutOfRangeException is a violation of Liskov. Pre-conditions cannot be strengthened: Assume your base class works with a member int. Now your sub-type requires that int to be positive. This is strengthened pre-conditions, and now any code that worked perfectly fine before with negative ints is broken. Post-conditions cannot be weakened: Assume your base class required all connections to the database should be closed before the method returned. In your sub-class you overrode that method and left the connection open for further reuse. You have weakened the post-conditions of that method. Invariants must be preserved: The most difficult and painful constraint to fulfill. Invariants are sometimes hidden in the base class and the only way to reveal them is to read the code of the base class. Basically you have to be sure when you override a method anything unchangeable must remain unchanged after your overridden method is executed. The best thing I can think of is to enforce these invariant constraints in the base class but that would not be easy. History Constraint: When overriding a method you are not allowed to modify an unmodifiable property in the base class. Take a look at these code and you can see Name is defined to be unmodifiable (private set) but SubType introduces new method that allows modifying it (through reflection): public class SuperType { public string Name { get; private set; } public SuperType(string name, int age) { Name = name; Age = age; } } public class SubType : SuperType { public void ChangeName(string newName) { var propertyType = base.GetType().GetProperty("Name").SetValue(this, newName); } }

还有2项:方法参数的逆变性和返回类型的协方差。但这在c#中是不可能的(我是c#开发人员)，所以我不关心它们。

LSP是关于类的契约的规则:如果基类满足契约，则LSP派生的类也必须满足该契约。

在Pseudo-python

class Base:
   def Foo(self, arg): 
       # *... do stuff*

class Derived(Base):
   def Foo(self, arg):
       # *... do stuff*

如果每次在派生对象上调用Foo，它给出的结果与在Base对象上调用Foo完全相同，只要arg是相同的。

到目前为止，我发现LSP最清晰的解释是“利斯科夫替换原则说，派生类的对象应该能够替换基类的对象，而不会给系统带来任何错误，也不会修改基类的行为”。文中给出了违反LSP的代码示例并进行了修复。

该原则由Barbara Liskov在1987年提出，并通过关注超类及其子类型的行为来扩展开闭原则。

当我们考虑违反它的后果时，它的重要性就变得显而易见了。考虑一个使用以下类的应用程序。

public class Rectangle 
{ 
  private double width;

  private double height; 

  public double Width 
  { 
    get 
    { 
      return width; 
    } 
    set 
    { 
      width = value; 
    }
  } 

  public double Height 
  { 
    get 
    { 
      return height; 
    } 
    set 
    { 
      height = value; 
    } 
  } 
}

想象一下，有一天，客户要求除了矩形之外还能操作正方形。因为正方形是矩形，所以square类应该派生自rectangle类。

public class Square : Rectangle
{
} 

然而，这样做会遇到两个问题:

一个正方形不需要从矩形继承高度和宽度变量，如果我们必须创建成千上万个正方形对象，这可能会造成严重的内存浪费。 从矩形继承的width和height setter属性不适用于正方形，因为正方形的宽度和高度是相同的。 为了将height和width设置为相同的值，我们可以创建两个新属性，如下所示:

public class Square : Rectangle
{
  public double SetWidth 
  { 
    set 
    { 
      base.Width = value; 
      base.Height = value; 
    } 
  } 

  public double SetHeight 
  { 
    set 
    { 
      base.Height = value; 
      base.Width = value; 
    } 
  } 
}

现在，当有人设置一个正方形物体的宽度时，它的高度将相应地改变，反之亦然。

Square s = new Square(); 
s.SetWidth(1); // Sets width and height to 1. 
s.SetHeight(2); // sets width and height to 2. 

让我们继续考虑另一个函数:

public void A(Rectangle r) 
{ 
  r.SetWidth(32); // calls Rectangle.SetWidth 
} 

如果我们将一个方形对象的引用传递给这个函数，我们将违反LSP，因为该函数对其参数的导数不起作用。属性width和height不是多态的，因为它们在矩形中没有被声明为虚的(正方形对象将被损坏，因为高度不会被改变)。

然而，通过将setter属性声明为virtual，我们将面临另一个违反，即OCP。事实上，派生类正方形的创建会导致基类矩形的变化。

LSP说“对象应该被它们的子类型替换”。 另一方面，这一原则指向

子类永远不应该破坏父类的类型定义。

通过以下示例，可以更好地理解LSP。

没有太阳能发电:

public interface CustomerLayout{

    public void render();
}


public FreeCustomer implements CustomerLayout {
     ...
    @Override
    public void render(){
        //code
    }
}


public PremiumCustomer implements CustomerLayout{
    ...
    @Override
    public void render(){
        if(!hasSeenAd)
            return; //it isn`t rendered in this case
        //code
    }
}

public void renderView(CustomerLayout layout){
    layout.render();
}

LSP修复:

public interface CustomerLayout{
    public void render();
}


public FreeCustomer implements CustomerLayout {
     ...
    @Override
    public void render(){
        //code
    }
}


public PremiumCustomer implements CustomerLayout{
    ...
    @Override
    public void render(){
        if(!hasSeenAd)
            showAd();//it has a specific behavior based on its requirement
        //code
    }
}

public void renderView(CustomerLayout layout){
    layout.render();
}