C# 中的接口和列表
我对以下接口和类有问题：
public interface IRelated
{
}
public class BaseItem:IRelated
{
public string Name{get;set;}
public List<IRelated> RelatedItems{get;set;}
}
现在，当我尝试在其他类中执行以下操作时，它会给我一个编译错误：
List<IRelated> listofrelateditems=new List<BaseItem>();
无法隐式转换类型
List到List
接口的原因是将来也许我会有另一个类可以与这个 BaseItem 相关。
im having issues with the following interface and a class:
public interface IRelated
{
}
public class BaseItem:IRelated
{
public string Name{get;set;}
public List<IRelated> RelatedItems{get;set;}
}
Now when i try to do in other classes the following it gives me a compilation error:
List<IRelated> listofrelateditems=new List<BaseItem>();
Cannot implicity convert type
List<BaseItem>toList<IRelated>
The reason of the interface is that in the future maybe i will have another class that can be Related to this BaseItem.
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评论（3）
你就是不能这样做 - 即使 .NET 4 中的通用协方差也无法帮助你，因为
List是一个类，甚至IList是不变的，因为它有T进来和出去。正是因为您将来可能会有一个新的
IRelated实现，所以您不能这样做。考虑一下：其中
OtherRelatedItem实现IRelated但不从BaseItem派生。现在您已经有了一个List，其中包含BaseItem以外的内容！换句话说，它破坏了类型安全。基本上，您必须创建一个
List而不是List。有关通用方差的更多信息以及为什么它有时适用有时不适用，请访问 NDC 2010 视频页面搜索“variance”即可找到我去年就该主题进行的演示的视频。
You just can't do that - even the generic covariance in .NET 4 won't help you, because
List<T>is a class and evenIList<T>is invariant as it hasTcoming "in" as well as going out.It's precisely because you might have a new implementation of
IRelatedin the future that you can't do that. Consider:where
OtherRelatedItemimplementsIRelatedbut doesn't derive fromBaseItem. Now you've got aList<BaseItem>which contains something other than aBaseItem! In other words, it breaks type safety.Basically you'll have to create a
List<IRelated>instead of aList<BaseItem>.For more on generic variance and why it's sometimes applicable and sometimes not, go to the NDC 2010 videos page and search for "variance" to find the video of a presentation I gave on the topic last year.
就像您自己所说：您可能有另一个可以是 IRelated 的类。如何将此类的实例添加到
List中？你必须写：
Like you yourself said: You may have another class that can be
IRelated. How are you going to add an instance of such a class to aList<BaseItem>?You will have to write:
更改为：
Change to: