Yes, they will be: GroupBy (MSDN). The IGrouping<TKey, TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey, TElement>. Elements in a grouping are yielded in the order they appear in source.
Further to the answers from mquander and Vasile Bujac, you could create a nice, straightforward singleton-esque EmptyLookup<K,E> class as follows. (In my opinion, there doesn’t seem much benefit to creating a full ILookup<K,E> implementation as per Vasile’s answer.) var empty = EmptyLookup<int, string>.Instance; // … public static class EmptyLookup<TKey, TElement> { private static readonly ILookup<TKey, … Read more
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 … Read more