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>
.