Linq To Objects - Under The Hood Of Joins

Question

I would like to know what are the differences between those two linq statements?

What is faster?

Are they the same?

What is the difference between this statement

from c in categories
from p in products
where c.cid == p.pid
select new { c.cname, p.pname };

and this statement?

from c in categories
join p in products on c.cid equals p.pid
select new { c.cname, p.pname };

Thanks in advance guys.

EDIT: In context of LINQ to Objects

Answer 1

Okay, within LINQ to Objects the difference can be very dramatic.

The first form examines every c and p pair, checks for c.cid being equal to p.pid and yields matches.

The second form (within Join) first creates a hash-based lookup from pid to matching Product elements. Then it streams the categories, and then checks for each category where there are matching Product elements in the lookup based on the c.cid. This is generally much more efficient as it only needs to look through products once and create the hash-based lookup. On the other hand, it has a higher memory footprint. This is all done somewhat lazily of course - it's only when you ask for the first result that anything significant happens.

For more details on the Join operation, see my Edulinq blog post on the topic.