问题描述
在的IDictionary＆LT; TKEY的，TValue＆GT; 在.NET 4 / Silverlight 4的不支持的协方差，即我不能做
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>();
模拟什么，我可以用的IEnumerable＆LT做; T＆GT; 取值现在
大概可以归结为 KeyValuePair＆LT; TKEY的，TValue＆GT; 不是协变要么。我觉得，协方差应该被允许在字典至少值。
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.
所以是一个错误或功能？他会不会来，也许在.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）：
将有一个<$c$c>IReadOnlyDictionary<TKey, TValue＆GT; 在.NET 4.5，但它不会是协变的既：·/ ，因为它派生自的IEnumerable＆LT; KeyValuePair＆LT; TKEY的，TValue＆GT;＆GT; 和 KeyValuePair＆LT; TKEY的，TValue＆GT; 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.
首创置业团队将不得不重新设计了许多上来，并使用一些 ICovariantPair＆LT; TKEY的，TValue＆GT; 代替。同时强类型的索引点菜此[TKEY的关键] 是不可能的协变接口。类似的下场只能放置一个扩展方法来实现的GetValue＆LT;＆GT;（这IReadOnlyDictionary＆LT; TKEY的，TValue＆GT;自我，TKEY的键）地方这会在一定程度内必须调用在实际实施中，这可以说是看起来像一个相当混乱的办法。
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
这是一个特点。 .NET 4.0只支持安全协方差。你提到的转换是潜在的危险，你可以添加一个非字符串元素，如果这是可能的字典：
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
在另一方面，的IEnumerable＆LT; T＆GT; 是一个只读接口。该 T 键入参数只在它的输出位置（当前属性的返回类型），所以它是安全的治疗的IEnumerable＆LT;字符串＆GT; 为的IEnumerable＆LT;对象＆gt;
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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