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

Create a new Dictionary<Color, Car> foundColors and a List<Car> carsToDelete

Then you iterate through your original list of cars like so:

foreach(Car c in listOfCars)
{
    if (foundColors.containsKey(c.Color))
    {
        carsToDelete.Add(c);
    }
    else
    {
        foundColors.Add(c.Color, c);
    }
}

Then you can delete every car that's in foundColors.

You could get a minor performance boost by putting your "delete record" logic in the if statement instead of creating a new list, but the way you worded the question suggested that you needed to collect them in a List.

Answer 2

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<TSource> DuplicatesBy<TSource, TKey>
    (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    HashSet<TKey> seenKeys = new HashSet<TKey>();
    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);