Benefits of manual lookup over joining in LINQ?

Question

Is constructing and using a manual lookup (ILookup<T>) approach any faster than using a join with Join or GroupJoin on a local IEnumerable<T> sequence in LINQ?

I read somewhere that the compiler actually translates the inner sequence of Join and GroupJoin to ILookup<T> anyway.

What would ILookup<T> benefits of using it on it's own be?

Answer 1

It depends. If you're working with objects that are DataContext based, then the join gets translated all the way down into the SQL server via the select statement. IF its a POCO (or more specifically a Plain Old CLR collection) or something else, then yes, its utilized as ILookup.