C#:没有来自 Class<Child> 的隐式转换到 Class<Base> [英] C#: No implict conversion from Class&lt;Child&gt; to Class&lt;Base&gt;

问题描述

以下代码段无法编译.出现以下错误:

Following snippet wouldn't compile. With following error:

无法隐式转换类型 'Container'到容器"

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>(); 
    }
}

这是故意的吗?如果是，原因是什么?

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

推荐答案

(制作 wiki，以防万一)

(made wiki, in case of dups)

C# (3.0) 不支持列表协变等.C# 4.0 将支持有限 [co|contra]variance，但 仍然不是列表.

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

问题在于:

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

我可以:

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

可以编译，但不能工作.

which would compile, but not work.

数组支持这种行为，但主要是出于历史原因.

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

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