C＃：从类< Child>没有隐式转换到类< Base> [英] C#: No implict conversion from Class<Child> to Class<Base>

问题描述

以下片段无法编译。出现以下错误：

无法将类型'Container< ChildClass>'隐式转换为'Container< BaseClass>'

  class BaseClass {} $ b $ class ChildClass：BaseClass {} $ b $ class Container< T> T：BaseClass {} 
 class Program {
 static void Main（）{
 //为什么这不起作用？ 
 Container< BaseClass> obj = new Container< ChildClass>（）; 
} 
} 
  

这是否由设计决定？如果是这样，原因是什么？

解决方案

b
C＃（3.0）不支持列表的协方差等。C＃4.0将支持限制 [co | contra]方差，但还没有列出。

问题在于：

  Container< BaseClass> obj = new Container< ChildClass>（）; 
  

我可以这样做：

  obj.Add（new SomeOtherSubclass（））; // SomeOtherSubclass：BaseClass 
  

可以编译但不能工作。

数组支持这种行为，但主要是出于历史原因。

Following snippet wouldn't compile. With following error:

Cannot implicitly convert type 'Container<ChildClass>' to 'Container<BaseClass>'

class BaseClass {}
class ChildClass : BaseClass {}
class Container<T> where T : BaseClass {}
class Program {
    static void Main() {
        // why doesn't this work?
        Container<BaseClass> obj = new Container<ChildClass>(); 
    }
}

Is this by design? If it is, what is the reason?

解决方案

(made wiki, in case of dups)

C# (3.0) doesn't support covariance of lists etc. C# 4.0 will support limited [co|contra]variance, but still not lists.

The problem is that with:

Container<BaseClass> obj = new Container<ChildClass>(); 

I could do:

obj.Add(new SomeOtherSubclass()); // SomeOtherSubclass : BaseClass

which would compile, but not work.

This behaviour is supported for arrays, but largely for historic reasons.

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