Get Non-Distinct elements from an IEnumerable

Question

I have a class called Item. Item has an identifier property called ItemCode which is a string. I would like to get a list of all non-distinct Items in a list of Items.

E

Answer 1

as an extension method:

public static IEnumerable NonDistinct (this IEnumerable source, Func keySelector)
{
   return source.GroupBy(keySelector).Where(g => g.Count() > 1).SelectMany(r => r);
}