为什么接口类型列表不能接受继承接口的实例？

Question

给出以下类型:

public interface IPrimary{ void doBattle(); }

// an ISecondary "is" an IPrimary
public interface ISecondary : IPrimary {  }

// An implementation of ISecondary is also an IPrimary:
internal class SecondaryImpl : ISecondary
{
    // Required, since this is an IPrimary
    public void doBattle(){ }
}

为什么我不这样做？

List<IPrimary> list = new List<ISecondary>();

这会导致以下编译错误:

参数类型'System.Collections.Generic.List'的参数类型不可分配给参数类型'System.Collections.Generic.List'

我理解错误,我意识到有一些解决方法.我只是没有看到任何明确的原因,为什么不允许这种直接转换.包含在列表中的值ISecondary,应毕竟是(通过扩展)类型的值IPrimary然后被.为什么List<IPrimary>和List<ISecondary>被解释为不相关的类型？

任何人都可以清楚地解释C#以这种方式设计的原因吗？

一个稍微扩展的例子:我在尝试执行类似以下操作时遇到了这个问题:

internal class Program
{
    private static void Main(string[] args)
    {
        // Instance of ISecondary, and by extention, IPrimary:
        var mySecondaryInstance = new SecondaryImpl();

        // This works as expected:
        AcceptImpl(mySecondaryInstance);

        // List of instances of ISecondary, which are also, 
        // by extention, instances of IPrimary:
        var myListOfSecondaries = new List<ISecondary> {mySecondaryInstance};

        // This, however, does not work (results in a compilation error):
        AcceptList(myListOfSecondaries);
    }

    // Note: IPrimary parameter:
    public static void AcceptImpl(IPrimary instance){  }

    // Note: List of type IPrimary:
    public static void AcceptList(List<IPrimary> list){  }

}

Answer 1

public class Animal
{
    ...
}

public class Cat: Animal
{
    public void Meow(){...}
}

List<Cat> cats = new List<Cat>();

cats.Add(new Cat());

cats[0].Meow();  // Fine.

List<Animal> animals = cats; // Pretend this compiles.

animals.Add(new Animal()); // Also adds an Animal to the cats list, since animals references cats.

cats[1].Meow(); // cats[1] is an Animal, so this explodes!

这就是原因.

Answer 2

为什么我不这样做？ List<IPrimary> list = new List<ISecondary>();

想象一下,你有一个像这样定义的方法:

public void PopulateList(List<IPrimary> listToPopulate)
{
    listToPopulate.Add(new Primary());  // Primary does not implement ISecondary!
}

如果你把它List<ISecondary>作为参数传递会发生什么？

List<ISecondary>无法分配List<IPrimary>的错误是编译器让您摆脱这些麻烦的方法.

Answer 3

class Evil : IPrimary {...}
list.Add(new Evil()); // valid c#, but wouldn't work

它可以保护您免受错误的影响.列表实例(对象)需要辅助实例.并非每个小学都是次要的.然而,期望的是,主要名单可以容纳任何主要名单.如果我们可以将二级列表视为主要列表:坏事.

实际上,数组确实允许这样做 - 如果你弄错的话,它会在运行时出错.