ImmutableSortedDictionary range enumeration by key

I was reading about C#'s ImmutableSortedDictionary in System.Collections.Immutable and thinking about how to apply it in my program. I quite like C++'s lower_bound and upper_bound (see here), and I was rather expecting to see something of the sort for range lookups. However, similar methods seem to be strangely absent from the documentation. Am I missing something? Or does MS truly provide a sorted dictionary without efficient access to the sorted ranges? That doesn't exactly seem like something one could do on an IEnumerable of the keys as say an extension method, so I'm a bit puzzled I'm not seeing something provided directly by the collection.