C#中的多态和使用静态方法的重载. [英] Polymorphism and overloading with static methods in C#.

问题描述

我一直在尝试生成一个工厂，该工厂应该根据函数getItem(A context)的输入参数(我称其为上下文)返回通用接口的另一个对象(例如Item)

I have been trying to generate a Factory supposed to return a different object of a common interface (say Item) according to the input parameter (I call it a context) of the function getItem(A context)

现在，假设我定义了一种新的上下文类型:B，它是从A继承的.

Now, assume I define a new type of context: B which inherits from A.

我想根据传递给工厂的对象是B还是A类返回不同的项目.

I wanted to return a different item depending on whether the object passed to the factory was of class B or A.

我尝试执行以下操作(重载方法):

I tried to do as follows (overloading the method):

class Factory
{
   static Item getItem(A context) {...}
   static Item getItem(B context) {...}
}

如果我做这样的事情，这会很好:

This works fine if I do something like this:

B bContext=new B();
Item it=Factory.getItem(bContext);

但是，如果我强制并拒绝键入A:

However, if I cast and object to type A:

A bContext=(A) new B();
Item it=Factory.getItem(bContext);

第一个工厂方法被调用.

the first factory method is called.

我认为即使在强制转换之后，多态性也可以确保第二种方法的执行，我想知道是否错过了什么吗?

I thought that polymorphism would ensure the execution of the second method even after the cast, and I would like to know if I missed something?

我知道我可以继续使用一个方法，并使用is运算符检查变量的类型，但是我认为我上面介绍的解决方案更加优雅.

I am aware that I could keep having a single method and use the is operator to check what the type of the variable is, but I thought the solution I presented above was a bit more elegant.

推荐答案

重载是根据编译时(除了在C#4中使用动态类型)决定的，参数-在您的最后一个代码段中，参数的编译时类型为A，因此调用了Factory.getItem(A).

Overloading is decided at compile-time (aside from using dynamic typing in C# 4) based on the compile-time type of the arguments - and in your last snippet, the compile-time type of the argument is A, so it calls Factory.getItem(A).

只有虚拟方法调用是多态的(使用 overriding )，在此方法中，目标对象的实际执行时间类型决定了要调用的实现.如果A和B拥有一个虚拟方法(在B中重写)是有意义的，可以由Factory.getItem调用该方法来处理差异，那就太好了...否则，您将无法使用任何动态方法或类似is的输入.

Only virtual method calls are polymorphic (using overriding), where the actual execution-time type of the target object to decide which implementation to call. If it makes sense for A and B to have a virtual method (overridden in B) which can be called by Factory.getItem to handle the differences, that's great... otherwise you're stuck with either dynamic typing or something like is.

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