C# 3.0: Need to return duplicates from a List<>

Question

I have a List<> of objects in C# and I need a way to return those objects that are considered duplicates within the list. I do not need the Distinct resultset, I need a

Answer 1

I inadvertently coded this yesterday, when I was trying to write a "distinct by a projection". I included a ! when I shouldn't have, but this time it's just right:

public static IEnumerable DuplicatesBy
    (this IEnumerable source, Func keySelector)
{
    HashSet seenKeys = new HashSet();
    foreach (TSource element in source)
    {
        // Yield it if the key hasn't actually been added - i.e. it
        // was already in the set
        if (!seenKeys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
}

You'd then call it with:

var duplicates = cars.DuplicatesBy(car => car.Color);