ILookup<TKey, TVal> vs. IGrouping<TKey, TVal>
c#, group-by, igrouping, ilookup, linq
Solution
Yes, all of those are correct.
And `ILookup<TKey, TValue>` also extends `IEnumerable<IGrouping<TKey, TValue>>` so you can iterate over all the key/collection pairs as well as (or instead of) just looking up particular keys.
I basically think of `ILookup<TKey,TValue>` as being like `IDictionary<TKey, IEnumerable<TValue>>`.
Bear in mind that `ToLookup` is a "do it now" operation (immediate execution) whereas a `GroupBy` is deferred. As it happens, with the way that "pull LINQ" works, when you start pulling `IGrouping`s from the result of a `GroupBy`, it has to read all the data anyway (because you can't switch group midway through) whereas in other implementations it may be able to produce a streaming result. (It does in Push LINQ; I would expect LINQ to Events to be the same.)
Problem
I've been having trouble articulating the differences between `ILookup<TKey, TVal>` and `IGrouping<TKey, TVal>`, and am curious if I understand it correctly now. LINQ compounded the issue by producing sequences of `IGrouping` items while also giving me a `ToLookup` extension method. So it felt like they were the same until I looked more closely. ``` var q1 = from n in N group n by n.MyKey into g select g; // q1 is IEnumerable<IGrouping<TKey, TVal>> ``` Which is equivalent to: ``` var q2 = N.GroupBy(n => n.MyKey, n => n); // q2 is IEnumerable<IGrouping<TKey, TVal>> ``` Which looks a lot like: ``` var q3 = N.ToLookup(n => n.MyKey, n => n); // q3 is ILookup<TKey, TVal> ``` Am I correct in the following analogies? - An `IGrouping<TKey, TVal>` is a single group (i.e. a keyed sequence), analogous to `KeyValuePair<TKey, TVal>` where the value is actually a sequence of elements (rather than a single element) - An `IEnumerable<IGrouping<TKey, TVal>>` is a sequence of those (similar to what you get when iterating over an `IDictionary<TKey, TVal>` - An `ILookup<TKey, TVal>` is more like a `IDictionary<TKey, TVal>` where the value is actually a sequence of elements