铸造名单＆LT;＆GT;的派生类清单及LT;＆GT;基类的 [英] Casting List<> of Derived class to List<> of base class

问题描述

我有两个类：一个基类（动物）和一个类从派生
它（CAT）.Base类包含一个虚方法是播放列表需要输入parameter.Something这样

I have two classes: a base class (Animal) and a class deriving from it (Cat).Base class contains one virtual method Play that takes List as input parameter.Something like this

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication9
{
    class Animal
    {
        public virtual void Play(List<Animal> animal) { }
    }
    class Cat : Animal
    {
        public override void Play(List<Animal> animal)
        {
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Cat cat = new Cat();
            cat.Play(new List<Cat>());
        }
    }
}

当我编译上面的程序，我收到以下错误

When i compile the above program,i get the following error

    Error    2    Argument 1: cannot convert from 'System.Collections.Generic.List' to 'System.Collections.Generic.List'

反正有没有做到这一点？

Is there anyway to accomplish this?

推荐答案

您不能这样做的原因是因为一个列表是可写的。假设它是合法的，看看有什么不顺心：

The reason you cannot do this is because a list is writable. Suppose it were legal, and see what goes wrong:

List<Cat> cats = new List<Cat>();
List<Animal> animals = cats; // Trouble brewing...
animals.Add(new Dog()); // hey, we just added a dog to a list of cats...
cats[0].Speak(); // Woof!

好狗猫我，这是不良。

Well dog my cats, that is badness.

要该功能被称为通用协方差，它在C＃4被支撑为已知是安全的接口。 的IEnumerable＆LT; T＆GT; 没有任何办法写序，因此它是安全的。

The feature you want is called "generic covariance" and it is supported in C# 4 for interfaces that are known to be safe. IEnumerable<T> does not have any way to write to the sequence, so it is safe.

class Animal    
{    
    public virtual void Play(IEnumerable<Animal> animals) { }    
}    
class Cat : Animal    
{    
    public override void Play(IEnumerable<Animal> animals) { }    
}    
class Program    
{    
    static void Main()    
    {    
        Cat cat = new Cat();    
        cat.Play(new List<Cat>());    
    }    
}  

这将工作在C＃4，因为列表＆LT;猫＆GT; 可转换为的IEnumerable＆LT;猫＆GT; ，这是转换为的IEnumerable＆LT;动物＆GT; 。有没有办法玩可以用的IEnumerable＆LT;动物方式＆gt; 来狗添加的东西，实际上是猫的列表

That will work in C# 4 because List<Cat> is convertible to IEnumerable<Cat>, which is convertible to IEnumerable<Animal>. There is no way that Play can use IEnumerable<Animal> to add a dog to something that is actually a list of cats.

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