IEqualityComparer for Intersect comparison not returning matching sets

Question

I have a class which has 3 properties and when it's being evaluated for equality, I only want 2 of them be accounted for. That means, in the following Product class, Misc should have no role in equality comparison.

However, when the code runs, only apple 9 a is displayed. But I also want apple 9 b be displayed too. So what is wrong in the code?

namespace ConsoleApplication1
{
    public class Product
    {
        public string Name { get; set; }
        public int Code { get; set; }
        public string Misc { get; set; }
    }

    class ProductComparer : IEqualityComparer<Product>
    {
        public bool Equals(Product x, Product y)
        {
            if (Object.ReferenceEquals(x, y)) return true;
            if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null)) return false;

            return x.Code == y.Code && x.Name == y.Name;
        }

        public int GetHashCode(Product product)
        {
            if (Object.ReferenceEquals(product, null)) return 0;
            int hashProductName = product.Name == null ? 0 : product.Name.GetHashCode();
            int hashProductCode = product.Code.GetHashCode();

            return hashProductName ^ hashProductCode;
        }

    }

    class Program
    {
        static void Main(string[] args)
        {
            Product[] store1 = { new Product { Name = "apple", Code = 9, Misc = "a" }, 
                                 new Product { Name = "orange", Code = 4 , Misc = "c"} };

            Product[] store2 = { new Product { Name = "apple", Code = 9, Misc = "b" }, 
                                 new Product { Name = "lemon", Code = 12, Misc = "d" } };

            IEnumerable<Product> duplicates = store1.Intersect(store2, new ProductComparer());

            foreach (var product in duplicates)
                Console.WriteLine(product.Name + " " + product.Code + " " + product.Misc);


            Console.Read();
        }
    }
}

score 2 · Accepted Answer · answered Oct 24 '14 at 19:40

To get the intersection of the two sets but to get the item from both sets, rather than just one of the matches, you'll need to use a Join. Intersect is just a special case of Join, you simply want to project the results of your matches into both results, rather than just one:

var duplicates = store1.Join(store2,
    p => p,
    p => p,
    (a, b) => new[] { a, b },
    new ProductComparer())
    .SelectMany(p => p);

score 1 · Answer 2 · answered Oct 24 '14 at 00:26

1

The reason why apple 9 b does not appear in the duplicates is that according to your Comparer, apple 9 b is same as apple 9 a. The result of Intersect only contains distinct values.

answered Oct 24 '14 at 00:26

sean717

11,759
20
66
90

I see, so I'll need inner join then. Correct? – Ray Cheng Oct 24 '14 at 05:05
if this is not for a technical interview...I think it is ok just to use a for loop and contains(). – sean717 Oct 24 '14 at 05:43
I found this one works for me and also generic enough. http://stackoverflow.com/questions/716552/can-you-create-a-simple-equalitycomparert-using-a-lamba-expression/716647#716647 – Ray Cheng Oct 24 '14 at 18:27

Dmitry · Answer 3 · 2014-10-24T18:59:29.313

0

You could find the duplicates again:

foreach (var product in store1.Concat(store2).Where(p => duplicates.Contains(p, new ProductComparer())))
    Console.WriteLine(product.Name + " " + product.Code + " " + product.Misc);

edited Oct 24 '14 at 18:59

answered Oct 24 '14 at 00:14

Dmitry

13,797
6
32
48

I still only displays just one. – Ray Cheng Oct 24 '14 at 18:39
@RayCheng Yup, I've updated the answer. Now the `ProductComparer` is used. Please revise. – Dmitry Oct 24 '14 at 19:04

IEqualityComparer for Intersect comparison not returning matching sets

3 Answers3