为什么不能接口类型的列表，接受继承接口intances？ [英] Why can't a list of an interface type accept intances of an inheriting interface?

问题描述

鉴于以下类型的

 公共接口IPRIMARY {无效doBattle（）; } 
 
 //一个ISecondary，是一个IPRIMARY 
公共接口ISecondary：IPRIMARY {} 
 
 // ISecondary的实现也是一个IPRIMARY：
内部类SecondaryImpl：ISecondary 
 {
 //必须的，因为这是一个IPRIMARY 
公共无效doBattle（）{} 
} 
  

我为​​什么不能这样做呢？

 列表<&IPRIMARY GT;名单=新名单，LT; ISecondary>（）; 
  

这将导致以下编译错误：

< BLOCKQUOTE>

参数类型'System.Collections.Generic.List'不是分配给参数类型'System.Collections.Generic.List

我理解错误，我知道有解决方法。我只是没有看到任何明确的理由，为什么这直接转换是不允许的。中包含的 ISecondary 列表值，毕竟应该是（按扩展名），键入 IPRIMARY 的值。那么，为什么是列表< IPRIMARY> 和列表< ISecondary> 被解释为不相关类型

任何人都可以解释清楚的C＃被设计成这样？

的稍微延长的理由例如：的试图做类似下面的东西，当我遇到问题就来了：

 内部类节目
 {
私有静态无效的主要（字串[] args）
 {
 // ISecondary的实例，并通过延伸，IPRIMARY：
变种mySecondaryInstance =新SecondaryImpl（）; 
 
 //这按预期工作：
 AcceptImpl（mySecondaryInstance）; 
 
。通过延伸ISecondary的实例，这也是，
 //的//列表，IPRIMARY的实例：
变种myListOfSecondaries =新列表与所述; ISecondary> {} mySecondaryInstance; 
 
 //但是，这不工作（编译错误结果）：
 AcceptList（myListOfSecondaries）; 
} 
 
 //注：IPRIMARY参数：
公共静态无效AcceptImpl（例如IPRIMARY）{} 
 
 //注：类型IPRIMARY一览：
公共静态无效AcceptList（列表< IPRIMARY>名单）{} 
 
} 
  

< DIV CLASS =h2_lin>解决方案

我为​​什么不能这样做呢？ 列表与LT; IPRIMARY>名单=新名单，LT; ISecondary>（）;

想象一下，你有过这样定义的方法：

 公共无效PopulateList（列表< IPRIMARY> listToPopulate）
 {
 listToPopulate.Add（新的主（ ））; //主没有实现ISecondary！ 
} 
  

如果你要传递一个会发生什么清单< ISecondary> 作为参数

这是错误的 列表< ISecondary> 不可转让从列表< IPRIMARY方式> 的是让你出这样的烦恼的编译器的方式

Given the following types:

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(){ }
}

Why can I not do this?

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

This results in the following compilation error:

Argument type 'System.Collections.Generic.List' is not assignable to parameter type 'System.Collections.Generic.List'

I understand the error, and I realize there are workarounds. I just do not see any clear reason why this direct conversion is disallowed. The values contained in a list of ISecondary, should after all, be (by extension) values of type of IPrimary.Why then are List<IPrimary> and List<ISecondary> being interpreted as unrelated types?

Can anyone explain clearly the reasoning for C# being designed this way?

A slightly extended example: I came across the issue when trying to do something similar to the following:

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){  }

}

解决方案

Why can I not do this? List<IPrimary> list = new List<ISecondary>();

Imagine that you had a method defined like this:

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

What would happen if you were to pass it a List<ISecondary> as a parameter?

The error that List<ISecondary> is not assignable from List<IPrimary> is the compiler's way of getting you out of such troubles.

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