IDictionary<TKey，TValue>在 .NET 4 中不是协变的 [英] IDictionary<TKey, TValue> in .NET 4 not covariant

问题描述

.NET 4/Silverlight 4 中的 IDictionary 不支持协方差，即我做不到

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>();

类似于我现在可以用 IEnumerable 做的事情.

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

可能归结为 KeyValuePair 也不是协变的.我觉得字典中应该允许协方差，至少对于这些值.

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.

这是错误还是功能?它会不会出现，也许在 .NET 37.4 中?

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

更新(2 年后):

将会有一个 IReadOnlyDictionary<.NET 4.5 中的 TKey, TValue>，但它也不会协变 :·/，因为它派生自 IEnumerable;> 和 KeyValuePair 不是接口，因此不能协变.

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东西才能提出并使用一些 ICovariantPair 代替.协变接口也不可能使用强类型索引器 á la this[TKey key].类似的结果只能通过将扩展方法 GetValue<>(this IReadOnlyDictionary<TKey, TValue> self, TKey key) 放置在某处以某种方式在内部必须调用实际实现来实现，它可以说看起来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 仅支持安全协变.您提到的演员表有潜在危险，因为您可以在可能的情况下向字典添加非字符串元素:

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 是一个只读接口.T 类型参数仅在其输出位置(Current 属性的返回类型)，因此可以安全地将 IEnumerable 视为IEnumerable