IEqualityComparer fails using two properties

Question

When I use this comparer in Distinct() it always returns false. Can't see a reason why.

public class IdEqualityComparer : IEqualityComparer<Relationship>
{
    public bool Equals(Relationship x, Relationship y)
    {
        if (x == null && y == null)
            return true;
        else if (x == null || y == null)
            return false;
        else if (x.ID == y.ID && x.RelatedID == y.RelatedID)
            return true;
        else
            return false;
    }

    public int GetHashCode(Relationship obj)
    {
        unchecked
        {
            int hash = (obj.ID ?? "").GetHashCode() ^ (obj.RelatedID ?? "").GetHashCode();
            return hash;
        }
    }
}

The hash seems correct to me, but the ID and RelatedID comparison never returns true.

It fails, as I can check the result afterward and the output is not distinct using those two properties.

`The hash seems correct to me, but the ID and RelatedID comparison never returns true.` <= please provide an [mcve] with a comparison between 2 instances that should return `true` but currently returns `false`. **Also remember to override `bool Equals(object other)` as well when you can't provide the IEqualityComparer!!** — Igor, Apr 06 '18 at 15:19
are you perhaps comparing objects where one is `""` and the other is `null`? this will cause the `GetHashCode()` to give the same value, but Equals` to return `false`. Note that this is **not** an invalid scenario - that's perfectly allowed. What *isn't* allowed is `Equals` returning true and `GetHashCode` returning *different* values. But ultimately, it would *really* help to see a full example here. — Marc Gravell, Apr 06 '18 at 15:24

score 1 · Answer 1 · answered Apr 06 '18 at 20:39

1

Apologies to all! It is working fine.

I was comparing objects defined similarly to Marc's answer. But, I did this:

var relators = relationships.Distinct(new RelationshipEqualityComparer());

and then sent relationships, rather than realtors, to the next method where the items were reviewed. Sometimes it takes another pair of eyes!

Thanks!

answered Apr 06 '18 at 20:39

Paul Keefe

44
3

great to hear that you figured it out – Marc Gravell Apr 06 '18 at 21:04

Marc Gravell · Accepted Answer · 2018-04-06T16:00:37.650

Seems to work fine here;

static void Main()
{
    var objs = new[]
    {
        new Relationship { ID = "a", RelatedID = "b" }, // <----\
        new Relationship { ID = "a", RelatedID = "c" }, //      |
        new Relationship { ID = "a", RelatedID = "b" }, // dup--/
        new Relationship { ID = "d", RelatedID = "b" }, // <------\
        new Relationship { ID = "d", RelatedID = "c" }, //        |
        new Relationship { ID = "d", RelatedID = "b" }, // dup ---/ 
        new Relationship { ID = "b", RelatedID = "c" }, //
    };

    var count = objs.Distinct(new IdEqualityComparer()).Count();
    System.Console.WriteLine(count);
}

gives 5, not 7 (which we would expect if it always returned false). Tested with:

public class Relationship
{
    public string ID { get; set; }
    public string RelatedID { get; set; }
}

To illustrate this more clearly:

var a = new Relationship { Id = "x", RelatedID = "foo" };
var b = new Relationship { Id = "y", RelatedID = "foo" };
var c = new Relationship { Id = "x", RelatedID = "foo" };

we can now demonstrate that the comparer returns true and false appropriately:

var comparer = new IdEqualityComparer();
Console.WriteLine(comparer.Equals(a, b)); // False
Console.WriteLine(comparer.Equals(a, c)); // True

we can also show that the hash-code is working appropriately:

Console.WriteLine(comparer.GetHashCode(a));
Console.WriteLine(comparer.GetHashCode(b));
Console.WriteLine(comparer.GetHashCode(c));

note that the numbers will change, but for me on this run this gives:

-789327704
1132350395
-789327704

The numbers don't matter - what matters is that the first and last are equal, and (ideally) different from the middle one.

So: the comparer is working fine, and the premise of the question is incorrect. You need to identify in your code what is different, and fix it.

IEqualityComparer fails using two properties

2 Answers2