在 C# 中，为什么不能 List<string>对象被存储在一个列表<对象>多变的 [英] In C#, why can't a List<string> object be stored in a List<object> variable

问题描述

It seems that a List object cannot be stored in a List variable in C#, and can't even be explicitly cast that way.

List<string> sl = new List<string>();
List<object> ol;
ol = sl;

results in Cannot implicitly convert type System.Collections.Generic.List<string> to System.Collections.Generic.List<object>

And then...

List<string> sl = new List<string>();
List<object> ol;
ol = (List<object>)sl;

results in Cannot convert type System.Collections.Generic.List<string> to System.Collections.Generic.List<object>

Of course, you can do it by pulling everything out of the string list and putting it back in one at a time, but it is a rather convoluted solution.

解决方案

Think of it this way, if you were to do such a cast, and then add an object of type Foo to the list, the list of strings is no longer consistent. If you were to iterate the first reference, you would get a class cast exception because once you hit the Foo instance, the Foo could not be converted to string!

As a side note, I think it would be more significant whether or not you can do the reverse cast:

List<object> ol = new List<object>();
List<string> sl;
sl = (List<string>)ol;

I haven't used C# in a while, so I don't know if that is legal, but that sort of cast is actually (potentially) useful. In this case, you are going from a more general class (object) to a more specific class (string) that extends from the general one. In this way, if you add to the list of strings, you are not violating the list of objects.

Does anybody know or can test if such a cast is legal in C#?

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