dictionary - IDictionary<TKey, TValue> in .NET 4 not covariant

Question

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>();

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

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.

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

UPDATE (2 years later):

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.

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.

Answer 1

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

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>.