Question

I suppose this doesn’t really matter, I’m just curious.

If the difference between dictionary and lookup is one is one-to-one and the other one-to-many, wouldn’t dictionary by a more specific/derived version of the other?

A lookup is a collection of key/value pairs where the key can be repeated.
A dictionary is a collection of key/value pairs where the key cannot be repeated.

Why couldn’t IDictionary implement ILookup?

Answer 1

I suspect this is mainly because the intention is different.

ILookup<T,U> is designed specifically to work with a collection of values. IDictionary<T,U> is intended to work with a single value (that could, of course, be a collection).

While you could, of course, have IDictionary<T,U> implementations implement this via returning an IEnumerable<U> with a single value, this would be confusing, especially if your “U” is a collection itself (ie: List<int>). In that case, would ILookup<T,U>.Item return an IEnumerable<List<int>>, or should it do some type of check for an IEnumerable<T> value type, and then “flatten” it? Either way, it’d look confusing, and add questionable value.