Quickest way to find the complement of two collections in C#

Question

I have two collections of type ICollection called c1 and c2. I\'d like to find the set of items that are in c2

Answer 1

If using C# 3.0 + Linq:

var complement = from i2 in c2
                 where c1.FirstOrDefault(i1 => i2.Id == i1.Id) == null
                 select i2;

Loop through complement to get the items.