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?
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)IEnumerable<IGrouping<TKey, TVal>>
is a sequence of those (similar to what you get when iterating over an IDictionary<TKey, TVal>
ILookup<TKey, TVal>
is more like a IDictionary<TKey, TVal>
where the value is actually a sequence of elementsYes, 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.)