有什么办法可以完成List< object> list = new List< int>（）in C＃？ [英] Is there any way to accomplish something like List<object> list = new List<int>() in C#?

问题描述

这是个问题。我想要完成的东西类似于下面的Java代码：

  class A {} 
 class B extends A {} 
 
 public class Tests {
 public static void main（String [] args）{
 ArrayList<延伸A> lists = new ArrayList< B>（）; 
 
 
 code $ <$ $ p 
 
 
 （其中 B扩展A 意味着B从A继承） 
 
 
 
有没有可能？
 
 
 
谢谢 
 
解决方案
尽管C＃4.0引入了接口方差的概念，但它不是（也不可能）专门做你所要求的。 
 
 
 
在C＃4.0中，您可以这样做： 
 
 
   IEnumerable<对象> foo = new List （）;   
 
 
 
 
 
 
编辑：正如Marc指出的那样，这不能通过值类型和引用类型来完成。然而，因为我认为你的问题更多的是关于一个广义的 A：B 而不是它是关于 int：object ，I认为重点在于。为了准确，我的意思是C＃4.0会允许 这个元素： 
 
 
  IEnumerable< ;对象> foo = new List< string>（）; 
  
 
 
 
 
 
 
但是你不能做你所描述的。也就是说，让我们考虑一下：
  List< int> foo = new List< int>（）; 
列表< object> bar = foo; 
 
 bar.Add（baz）; 
  
现在，当我们尝试访问 foo [0] ？你现在已经破坏了列表中的类型安全。
 
That is the question. What I want to accomplish is something similar to the following Java code:
class A { }
class B extends A { }

public class Tests {
    public static void main(String [] args) {
        ArrayList<? extends A> lists = new ArrayList<B>(); 
    }
}
(in which B extends A means B inherits from A)

Is it possible at all?

Thanks
解决方案
No. While C# 4.0 introduces the concept of variance for interfaces, it isn't (and can't be) possible to do what you're asking specifically. 

In C# 4.0, you can do this:

IEnumerable<object> foo = new List<int>();



Edit: As Marc points out, this can't be done with value types and reference Types. However, since I think your question was more about a generalized A:B than it was about int:object, I think the point carries. To be accurate, though, what I mean is that C# 4.0 will allow something this:
IEnumerable<object> foo = new List<string>();




But you can't do what you describe. Namely, let's consider:
List<int> foo = new List<int>();
List<object> bar = foo;

bar.Add("baz");
Now, what happens when we try to access foo[0]? You've now broken the type safety on the list.

                        
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