Two objects being equal not regarded as distinct by LINQ

Question

I'm comparing two objects of type Triangle and apparently they are deemed equal (I implemented my custom GetHaschCode as well as Equal method and operator).

List<Triangle> triangles = ...;
bool same = triangles[0] == triangles[1];
// same is true

However, when I go Distinct() on that list, it keeps all the elements (which sound to me like it's comparing by reference and not by my custom conditions). Is it so and what can I do about it?

int countBefore = triangles.Count();
int countAfter  = triangles.Distinct().Count();
bool same = countBefore == countAfter;
// same is true, again

I'm missing something fairly obvious, am I not?

Hmm, can you show a reproducible example? Sounds like incorrect `GetHashCode` implementation. — Ivan Stoev, Sep 08 '16 at 19:39
It will not help. If you have **correctly** implemented `Equals` and `GetHashCode` overrides in your `Triangle` class, `Distinct` would work correctly w/o `IEqualtable`. If your `GetHasCode` is incorrect, implementing the interface will not change anything. Just post the class code and we'll tell you what's the problem. See, if you return different hash codes, your `Equals` method will not be called at all. — Ivan Stoev, Sep 08 '16 at 19:50
@IvanStoev Please post that as reply so I can accept it. You're spot on correct, mate. You can mention something from my comment above too to make the answer even more useful for the future generations. Genius-ishly! — Konrad Viltersten, Sep 08 '16 at 20:15
@KonradViltersten Thank you mate, but I've seen this many times and I'm pretty sure it's a dupe. For instance, http://stackoverflow.com/questions/371328/why-is-it-important-to-override-gethashcode-when-equals-method-is-overridden does explain all that. — Ivan Stoev, Sep 08 '16 at 20:22
@IvanStoev It seems so, yes. In fact, I can see I upvote some of the answers there so I **must have seen it**. Yet,didn't made the connection, hehe. — Konrad Viltersten, Sep 08 '16 at 20:26

score 1 · Answer 1 · answered Sep 08 '16 at 19:50

1

You can do this in one of two ways...

Either as Andrew says implement IEquatable

public class Triangle : IEquatable<Triangle>
{
    bool IEquatable<Triangle>.Equals(Triangle other)
    {
        return Equals(other);
    }

    public override bool Equals(object obj)
    {
        //...
    }

    public override int GetHashCode()
    {
        //...
    }
}

Or you could create another class that implements IEqualityComparer(T) and pass that into the Distinct method call.

public class TriangleComparer : IEqualityComparer<Triangle>
{
    public bool Equals(Triangle x, Triangle y)
    {
        return x.Equals(y);
    }

    public int GetHashCode(Triangle obj)
    {
        return obj.GetHashCode();
    }
}

answered Sep 08 '16 at 19:50

Scott Perham

2,410
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I wonder if it's supposed to be *bool IEquatable.Equals(Triangle other)* or just *bool Equals(Triangle other)* like Resharper suggests. – Konrad Viltersten Sep 08 '16 at 19:52
It depends if you want to be able to call the method by having to cast to the interface first (explicit implementation) or not (implicit implementation) - It does also depend on the usage, but as it was only going to be used in the Distinct call it probably would make sense to implement it explicitly – Scott Perham Sep 08 '16 at 20:00

Andrew Jenkins · Answer 2 · 2016-09-08T19:53:00.417

0

You need to implement the IEquatable(T) interface in your Triangle class.

From the Enumerable.Distinct() documentation:

The default equality comparer, Default, is used to compare values of the types that implement the IEquatable generic interface. To compare a custom data type, you need to implement this interface and provide your own GetHashCode and Equals methods for the type.

edited Sep 08 '16 at 19:53

answered Sep 08 '16 at 19:36

Andrew Jenkins

1,590
13
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I do have *public override bool Equals(object input)* already. Isn't that sufficient? Is it perhaps superfluous to have both that and *public bool Equals(Triangle other)*? – Konrad Viltersten Sep 08 '16 at 19:48
I updated my answer. As for why this is the case in C#, check out http://stackoverflow.com/a/2734941/743868 – Andrew Jenkins Sep 08 '16 at 19:55
2

Well, the documentation is just incorrect. If the class does **not** implement `IEquatable`, the default equality comparer uses `object.Equals` and `object.GetHashCode` overrides. See `ObjectEqualityComparer` in the [source](http://referencesource.microsoft.com/#mscorlib/system/collections/generic/equalitycomparer.cs,542680fa5b2d828d). – Ivan Stoev Sep 08 '16 at 19:56
I'm afraid there's something more wrong. I implemented *IEquatable* in my *Triangle* class and added the second *Equal* method as Resharper suggested. Same result, though... – Konrad Viltersten Sep 08 '16 at 19:57
Strange, I just tested it and am getting the same result as Ivan and Ed. There is probably something wrong with your Equals or GetHashCode method. – Andrew Jenkins Sep 08 '16 at 20:10
Btw, it's better documented here - [EqualityComparer.Default Property](https://msdn.microsoft.com/en-us/library/ms224763(v=vs.110).aspx) - **Remarks** section: *The Default property checks whether type T implements the System.IEquatable interface and, if so, returns an EqualityComparer that uses that implementation. Otherwise, it returns an EqualityComparer that uses the overrides of Object.Equals and Object.GetHashCode provided by T.* – Ivan Stoev Sep 08 '16 at 20:16

Two objects being equal not regarded as distinct by LINQ

2 Answers2