IDictionary< TKey，TValue>在.NET 4中不协调 [英] IDictionary<TKey, TValue> in .NET 4 not covariant

问题描述

.NET 4 / Silverlight 4中的 IDictionary< TKey，TValue> 不支持协方差，即我不能执行

The IDictionary<TKey, TValue> in .NET 4 / Silverlight 4 does not support covariance, i.e. I can't do a

IDictionary<string, object> myDict = new Dictionary<string, string>();

模拟我可以用 IEnumerable< T> s现在。

analog to what I can do with IEnumerable<T>s now.

可能归结为 KeyValuePair< TKey，TValue> 也不是协变的。我觉得在词典中应该允许协方差至少为这些值。

Probably boils down to the KeyValuePair<TKey, TValue> not being covariant either. I feel that covariance should be allowed in dictionaries at least for the values.

那么是一个bug还是一个功能？是否会来，可能在.NET 37.4？

So is that a bug or a feature? Will it ever come, maybe in .NET 37.4?

更新（2年后）：

成为 IReadOnlyDictionary< TKey， .NET5.5中的TValue> ，但它不会是：·/ ，因为它来源于 IEnumerable< KeyValuePair< TKey，TValue> 和 KeyValuePair< TKey，TValue> 不是界面，因此不能是协变

There will be an IReadOnlyDictionary<TKey, TValue> in .NET 4.5, but it won't be covariant either :·/, because it derives from IEnumerable<KeyValuePair<TKey, TValue>>, and KeyValuePair<TKey, TValue> is not an interface and thus cannot be covariant.

BCL团队必须重新设计很多，并使用一些 ICovariantPair&TK。，TValue> 代替。对于协变接口，强大类型的索引器也不可能使用[TKey key] 。类似的结局只能通过在内部必须调用的某个地方放置一个扩展方法 GetValue（this IReadOnlyDictionary< TKey，TValue> self，TKey key）一个实际的实现，可以说是一个相当混乱的方法。

The BCL team would have to redesign a lot to come up and use some ICovariantPair<TKey, TValue> instead. Also strongly-typed indexers á la this[TKey key] aren't possible for covariant interfaces. A similar end can only be achieved by placing an extension method GetValue<>(this IReadOnlyDictionary<TKey, TValue> self, TKey key) somewhere which would somehow internally have to call an an actual implementation, which arguably looks like a quite messy approach.

推荐答案

这是一个功能。 .NET 4.0仅支持安全性协方差。你提到的演员有潜在的危险，因为你可以添加非字符串元素到字典，如果可能的话：

It's a feature. .NET 4.0 only supports safe covariance. The cast you mentioned is potentially dangerous as you could add a non-string element to the dictionary if that was possible:

IDictionary<string, object> myDict = new Dictionary<string, string>();
myDict["hello"] = 5; // not an string

另一方面， IEnumerable< T& / code>是一个只读接口。 T type参数只在其输出位置（ Current 属性的返回类型），因此， code> IEnumerable< string> 作为 IEnumerable< object> 。

On the other hand, IEnumerable<T> is a read-only interface. The T type parameter is only in its output positions (return type of the Current property) so it's safe to treat IEnumerable<string> as an IEnumerable<object>.

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