C# 泛型和多态性:矛盾吗? [英] C# Generics and polymorphism: an oxymoron?

问题描述

我只是想确认一下我对 C# 中的泛型的理解.这出现在我工作过的几个代码库中，其中通用基类用于创建类型安全的派生实例.我所说的一个非常简单的例子，

I just want to confirm what I've understood about Generics in C#. This has come up in a couple code bases I've worked in where a generic base class is used to create type-safe derived instances. A very simple example of what I'm talking about,

public class SomeClass<T>
{
    public virtual void SomeMethod(){ }
}

public class DeriveFrom :SomeClass<string>
{
    public override void SomeMethod()
    {
        base.SomeMethod();
    }
}

当我想以多态方式使用派生实例时，问题就出现了.

The problem comes up when I then want to use derived instances in a polymorphic way.

public class ClientCode
{
    public void DoSomethingClienty()
    {
        Factory factory = new Factory();
        //Doesn't compile because SomeClass needs a type parameter!
        SomeClass someInstance = factory.Create();

        someInstance.SomeMethod();
    }
}

似乎一旦将 Generic 引入继承层次结构或接口，您就不能再以多态方式使用该类族，除非它可能是其内部的.这是真的吗?

It seems that once you introduce a Generic into an inheritance hierarchy or interface, you can no longer use that family of classes in a polymorphic way except perhaps internal to itself. Is that true?

推荐答案

据我所知，消费代码不需要泛型类的细节(即它不依赖于什么T是).那么，为什么不引入 SomeClass<T> 将实现的接口，并使用该接口的实例.

As far as I can see, consuming code doesn't need specifics of generic class (i.e., it doesn't depends on what T is). So, why don't you introduce interface that SomeClass<T> will implement, and use instance of this interface.

例如:

public interface ISome
{
    void SomeMethod();
}

public class SomeClass<T>: ISome
{
    public virtual void SomeMethod(){ }
}

public void DoSomethingClienty()
{
    Factory factory = new Factory();
    ISome someInstance = factory.Create();

    someInstance.SomeMethod();
}

现在，SomeClass<T> 的子类可以在不同的 T 上进行不同的操作，但使用代码不会改变.

Now, subclasses of SomeClass<T> can operate differently on different Ts, but consuming code won't change.

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