当值类型可以相互转换时，为什么我不能将一种值类型的字典转换为另一种值类型的字典? [英] Why can't I cast a dictionary of one value type to dictionary of another value type when the value types can be cast from one another?
问题描述
可能的重复:
在 C# 中，为什么不能't a List
为什么下面的方法不起作用?
Why doesn't the below work?
List<string> castMe = new List<string>();
IEnumerable<string> getFromCast = (IEnumerable<string>)castMe; // allowed.
Dictionary<int, List<string>> castMeDict = new Dictionary<int, List<string>>();
Dictionary<int, IEnumerable<string>> getFromDict = (Dictionary<int, IEnumerable<string>>)castMeDict; // Not allowed
这是 Dictionary 转换机制的缺陷，还是我认为应该允许这样做?
Is this a flaw in the Dictionary casting mechanism, or in my thinking that this should be allowed?
谢谢.
推荐答案
这是字典转换机制的缺陷，还是我认为应该允许这样做?
Is this a flaw in the Dictionary casting mechanism, or in my thinking that this should be allowed?
在你的想法中.您期望字典在它们的转换中应该协变.它们不是，原因如下.假设它们是，并推断出可能出错的地方:
In your thinking. You are expecting that dictionaries should be covariant in their conversions. They are not, for the following reason. Suppose they were, and deduce what could go wrong:
Dictionary<int, List<string>> castMeDict =
new Dictionary<int, List<string>>();
Dictionary<int, IEnumerable<string>> getFromDict =
(Dictionary<int, IEnumerable<string>>)castMeDict;
castMeDict[123] = new List<string>();
IEnumerable<string> strings = getFromDict[123]; // No problem!
getFromDict[123] = new string[] { "hello" }; // Big problem!
字符串数组可转换为IEnumerable，但不能转换为List.您只需将不是字符串列表的内容放入只能取字符串列表的字典中.
An array of string is convertible to IEnumerable<string> but not to List<string>. You just put something that is not a list of string into a dictionary that can only take list of string.
在 C# 中，如果满足以下所有条件，泛型类型可能是协变的或逆变的:
In C# generic types may be covariant or contravariant if all the following conditions are met:
- 您使用的是 C# 4 或更高版本.
- 不同的泛型类型是接口或委托.
- 可证明该方差是类型安全的.(C# 规范描述了我们用来确定方差安全的规则.可以下载 C# 4.0 版本的文档文件 [此处].参见第 23.5 节.)
- 变化的类型参数都是引用类型.
- 该类型已被明确标记为安全的变异.
- You're using C# 4 or better.
- The varying generic type is an interface or delegate.
- The variance is provably typesafe. (The C# specification describes the rules we use to determine variance safety. C# 4.0 Version doc file can be downloaded [here]. See section 23.5.)
- The type arguments that vary are all reference types.
- The type has been specifically marked as safe for variance.
字典不满足这些条件中的大多数——它不是接口或委托，它不能证明是安全的，并且类型没有标记为安全的变化.因此，字典没有差异.
Most of those conditions are not met for dictionary -- it is not an interface or delegate, it is not provably safe, and the type is not marked as safe for variance. So, no variance for dictionaries.
IEnumerable 确实满足所有这些条件.您可以在 C# 4 中将 IEnumerable 转换为 IEnumerable.
IEnumerable<T> by contrast does meet all those conditions. You can convert IEnumerable<string> to IEnumerable<object> in C# 4.
如果您对差异主题感兴趣，请考虑阅读我关于该主题的两打文章:
If the subject of variance interests you, consider reading my two dozen articles on the subject:
http://blogs.msdn.com/b/ericlippert/archive/标签/协方差+和+逆变/
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