Why are ToLookup and GroupBy different?

Question

.ToLookup returns an ILookup. ILookup also implements interface I

Answer 1

The two are similar, but are used in different scenarios. .ToLookup() returns a ready to use object that already has all the groups (but not the group's content) eagerly loaded. On the other hand, .GroupBy() returns a lazy loaded sequence of groups.

Different LINQ providers may have different behaviors for the eager and lazy loading of the groups. With LINQ-to-Object it probably makes little difference, but with LINQ-to-SQL (or LINQ-to-EF, etc.), the grouping operation is performed on the database server rather than the client, and so you may want to do an additional filtering on the group key (which generates a HAVING clause) and then only get some of the groups instead of all of them. .ToLookup() wouldn't allow for such semantics since all items are eagerly grouped.