The .NET type
SortedDictionary<TKey, TValue> does not implement the interface
IReadOnlyDictionary<TKey, TValue>, although
Dictionary<TKey, TValue> does.
It is trivial to make
IReadOnlyDictionary by subclassing it (< 10 LOCs).
But I guess there are good reasons why the framework developers did not do it and that’s why I shouldn’t do it. What are those reasons?