Why doesn't IDictionary <TKey, TValue> implement IReadOnlyDictionary <TKey, TValue>?

I noticed that the Dictionary class implements IDictionary<TKey, TValue>

as well IReadOnlyDictionary<TKey, TValue>

.

However, the IDictionary interface does not implement IReadOnlyDictionary<TKey, TValue>

.

This means that if my class has IDictionary<TKey, TValue>

, and I need to pass IReadOnlyDictionary<TKey, TValue>

, I have to create an adapter for that, or use some other job.

IMHO every class that has functionality has functionality IDictionary<TKey, TValue>

as well IReadOnlyDictionary<TKey, TValue>

.

Did they just forget to implement it, or are there really meaningful classes that implement IDictionary<TKey, TValue>

that can't have meaningful implementations IReadOnlyDictionary<TKey, TValue>

?