我有一个抽象类,有一些继承类,如:
internal abstract class SomeBaseType {}
internal class FirstSpecificType : SomeBaseType {}
internal class SecondSpecificType : SomeBaseType {}
internal class ThirdSpecificType : SomeBaseType {}
和具有约束泛型类型参数的类:
internal class SomeCollection<T> : ICollection<SomeDataStructure>
where T : SomeBaseType {}
我创建了List<SomeCollection<SomeBaseType>>
,并尝试添加SomeBaseType
的各种继承类的元素,但我收到以下错误(CS1503和CS1950):
Argument 1: cannot convert from 'Namespace.SomeCollection<FirstSpecificType>' to 'Namespace.SomeCollection<SomeBaseType>'
The best overloaded Add method 'List<SomeCollection<SomeBaseType>>.Add(SomeCollection<SomeBaseType>)' for the collection initializer has some invalid arguments
由于FirstSpecificType
是SomeBaseType
,我应该可以这样做,对吧?它与向object
添加一些IList<object>
的任意一组基本相同。
对于其他上下文,此错误显示的代码如下所示:
protected Constructor()
{
collectionOne = new SomeCollection<FirstSpecificType>();
collectionTwo = new SomeCollection<SecondSpecificType>();
collectionThree = new SomeCollection<ThirdSpecificType>();
allCollections = new List<SomeCollection<SomeBaseType>>
{
// Each of these three collections has the error
collectionOne,
collectionTwo,
collectionThree
};
}
答案 0 :(得分:1)
这是covariance and contravariance的问题,基本上说虽然
FirstSpecificType
确实是SomeBaseType
,但SomeCollection<FirstSpecificType>
严格来说不是 {{1 }}
Eric Lippert写了一些关于这个和长颈鹿的精彩articles,我建议大家至少阅读一次: - )