Elegant way to combine multiple collections of elements?

Question

Say I have an arbitrary number of collections, each containing objects of the same type (for example, List foo and List bar).

Answer 1

All you need is this, for any IEnumerable> lists :

var combined = lists.Aggregate((l1, l2) => l1.Concat(l2));

This will combine all the items in lists into one IEnumerable (with duplicates). Use Union instead of Concat to remove duplicates, as noted in the other answers.