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 list of those items that I will be deleting from my repository.

For the sake of this example, lets say I have a list of "Car" types and I need to know which of these cars are the same color as another in the list. Here are the cars in the list and their color property:

Car1.Color = Red;

Car2.Color = Blue;

Car3.Color = Green;

Car4.Color = Red;

Car5.Color = Red;

For this example I need the result (IEnumerable<>, List<>, or whatever) to contain Car4 and Car5 because I want to delete these from my repository or db so that I only have one car per color in my repository. Any help would be appreciated.

Answer 1

IEnumerable<Car> GetDuplicateColors(List<Car> cars)
{
    return cars.Where(c => cars.Any(c2 => c2.Color == c.Color && cars.IndexOf(c2) < cars.IndexOf(c) ) );
}    

It basically means "return cars where there's any car in the list with the same color and a smaller index".

Not sure of the performance, though. I suspect an approach with a O(1) lookup for duplicates (like the dictionary/hashset method) can be faster for large sets.

Answer 2

My answer takes inspiration (in this order) from the followers respondents: Joe Coehoorn, Greg Beech and Jon Skeet.

I decided to provide a full example, with the assumption being (for real word efficiency) that you have a static list of car colors. I believe the following code illustrates a complete solution to the problem in an elegant, although not necessarily hyper-efficient, manner.

#region SearchForNonDistinctMembersInAGenericListSample
public static string[] carColors = new[]{"Red", "Blue", "Green"}; 
public static string[] carStyles = new[]{"Compact", "Sedan", "SUV", "Mini-Van", "Jeep"}; 
public class Car
{
    public Car(){}
    public string Color { get; set; }
    public string Style { get; set; }
}
public static List<Car> SearchForNonDistinctMembersInAList()
{
    // pass in cars normally, but declare here for brevity
    var cars = new List<Car>(5) { new Car(){Color=carColors[0], Style=carStyles[0]}, 
                                      new Car(){Color=carColors[1],Style=carStyles[1]},
                                      new Car(){Color=carColors[0],Style=carStyles[2]}, 
                                      new Car(){Color=carColors[2],Style=carStyles[3]}, 
                                      new Car(){Color=carColors[0],Style=carStyles[4]}};
    List<Car> carDupes = new List<Car>();

    for (int i = 0; i < carColors.Length; i++)
    {
        Func<Car,bool> dupeMatcher = c => c.Color == carColors[i];

        int count = cars.Count<Car>(dupeMatcher);

        if (count > 1) // we have duplicates
        {
            foreach (Car dupe in cars.Where<Car>(dupeMatcher).Skip<Car>(1))
            {
                carDupes.Add(dupe);
            }
        }
    }
    return carDupes;
}
#endregion

I'm going to come back through here later and compare this solution to all three of its inspirations, just to contrast the styles. It's rather interesting.