The .NET type SortedDictionary<TKey, TValue> does not implement the interface IReadOnlyDictionary<TKey, TValue>, although Dictionary<TKey, TValue> does.

It is trivial to make SortedDictionary implement 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?

  • Just a guess, though maybe SortedDictionary optimizes sorting by sorting only upon retrieving data, so you can insert many values without sorting each time. However in doing so, reading a value is actually changing the dictionary's state, hence it isn't read only. This is just my idea, so I won't post it as a solution.
    – Neil
    Commented Sep 22, 2014 at 8:52
  • 2
    I guess they simply forgot to add it, or considered it too much work for too little gain. Commented Sep 22, 2014 at 8:58
  • @Neil: Good idea! However, they are using red-black trees, and the code does not seem to modify the collection. Moreover, the docs says about thread safety: “A SortedDictionary<TKey, TValue> can support multiple readers concurrently, as long as the collection is not modified.” So I think that’s not the reason.
    – Martin
    Commented Sep 22, 2014 at 9:04
  • @Martin It's reasonable to assume there is some detail like that going on though. Otherwise I can't see why they wouldn't make SortedDictionary implement IReadOnlyDictionary.
    – Neil
    Commented Sep 22, 2014 at 9:51
  • Hmmm. Also looks like SortedList doesn't support IReadOnlyList.
    – J Trana
    Commented Sep 24, 2014 at 1:58

1 Answer 1


Looking at MSDN today, one can see that the MSDN documentation page for SortedDictionary<TKey, TValue>, SortedDictionary<TKey, TValue> does implement IReadOnlyDictionary<TKey, TValue>.

According to the internet archive, that was not the case on November 14th, 2013.

Given that information, it is likely that it was simply an oversight from Microsoft, fixed in a recent .NET point update.

  • Indeed. The MSDN page mentions “.NET 4.5.3”, and today, sourceof.net shows 4.5.2, where IReadOnlyDictionary is not implemented.
    – Martin
    Commented Dec 2, 2014 at 11:35

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