LINQ group by

This one caught me out a couple of days ago. Recall to mind the good old IEnumerable . It's a good idea to act and code as if they can be traversed once only. As it happened we had some test code which iterated through two streams. Foolishly and without applying enough thought we wrote some linq which joined the two datastreams (ok so far) and then grouped them. Var a = from b in Bs From c in Cs Select new { b , c}; Var aa = from j in a group j by j.c.name; Oh dear. The group caused the Cs to be looped through one complete traversal for each element in Bs! Not good, because the stream was not restartable and so it went horribly wrong. The temporary solution is to turn Bs and Cs into lists and take the cross-product on the chin for now.