IEqualityComparer not working at all

Question

I have been trying to make my equality definition work but found out that IEqualityComparer does not seem to work whatsoever.

My class:

public class DBTileSimple
{
    public int X;
    public int Y;
    public int Zoom;

    public DBTileSimple(int x, int y, int z)
    {
        X = x;
        Y = y;
        Zoom = z;
    }
}

Testing IEqualityComparer, so it should be equal for any object:

public class TileComparer : IEqualityComparer<DBTileSimple>
{
    public bool Equals(DBTileSimple x, DBTileSimple y)
    {
        return true;
    }

    public int GetHashCode(DBTileSimple obj)
    {
        return 1;
    }
}

Result:

DBTileSimple t1 = new DBTileSimple(10, 20, 17);
DBTileSimple t2 = new DBTileSimple(10, 20, 17);
Log.Info("t1 and t2 = " + (t1 == t2));

returns t1 and t2 = false

My ultimate goal is to compare two lists with these objects and do logical operations over them (intersection, etc.).

So for example:

DBTileSimple t1 = new DBTileSimple(10, 20, 17);
DBTileSimple t2 = new DBTileSimple(10, 20, 17);

List<DBTileSimple> list1 = new List<DBTileSimple>();
list1.Add(t1);

List<DBTileSimple> list2 = new List<DBTileSimple>();
list2.Add(t2);

list1 = list1.Except(list2).ToList();

Now list1 should be empty but it is not.

You aren't using your comparer at all when using `==`. If you want that to work you need to add an overload for `operator ==`. — Lee, Sep 01 '17 at 08:53
`==` will not call `object.Equals`. Thus you should override it also and let it call `Equals` (or vice versa). — MakePeaceGreatAgain, Sep 01 '17 at 08:56
== operator doesn't use IEqualityComaprer. You need to override Equals method in DBTitleSimple class. — Chetan, Sep 01 '17 at 08:57
Possible duplicate of [Operator overloading ==, !=, Equals](https://stackoverflow.com/questions/25461585/operator-overloading-equals) — mjwills, Sep 01 '17 at 08:59
it also doesn't work when working with lists. List list1 = new List(); list1.Add(t1); List list2 = new List(); list1.Add(t2); list1 = list1.Except(list2).ToList(); list1 should be now empty but is not.. — Michal Krča, Sep 01 '17 at 09:02
You need to implement `GetHashCode` as well - see the link I provided. — mjwills, Sep 01 '17 at 10:04

René Vogt · Accepted Answer · 2019-06-12T12:20:07.080

You don't use the TileComparer at all. If you want to use it to compare to DBTileSimple instances you can do it like that:

DBTileSimple t1 = new DBTileSimple(10, 20, 17);
DBTileSimple t2 = new DBTileSimple(10, 20, 17);
TileComparer comparer = new TileComparer();
Log.Info("t1 and t2 = " + comparer.Equals(t1, t2));

If you want t1 == t2 to work, you have to overload that operator:

public class DBTileSimple
{
    //...

    public static bool operator==(DBTileSimpe t1, DBTileSimpe t2)
    {
        return true;
    }
    public static bool operator!=(DBTileSimpe t1, DBTileSimpe t2)
    {
        return false;
    }
}

Note that you need to overload both == and !=, see Microsoft's guidelines.

UPDATE after your edit:

To use your comparer with Except you simply need to pass an instance of it:

list1 = list1.Except(list2, new TileComparer()).ToList();

score 0 · Answer 2 · answered Sep 01 '17 at 09:00

0

You need to implement == operator like this:

public static bool operator ==(DBTileSimple a, DBTileSimple b)
{    
    // Return true if the fields match:
    return a.x == b.x && a.y == b.y && a.Zoom == b.Zoom;
}

public static bool operator !=(DBTileSimple a, DBTileSimple b)
{
    return !(a == b);
}

answered Sep 01 '17 at 09:00

Jalal

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@MichalKrča So as @Rene mentioned in his answer, you need to pass an instance of `TileComparer` to `Except` method. – Jalal Sep 01 '17 at 09:21

Kristianne Nerona · Answer 3 · 2017-09-01T09:17:59.007

Overriding the Equals() will have an effect only with

t1.Equals(t2)

To make your code work, you have to add this code in the DBTileSimple class:

public static bool operator ==(DBTileSimple x, DBTileSimple y)
{
    return x.propA == y.propB;
}

Please check this link for more information about overriding equality:

https://msdn.microsoft.com/en-US/library/ms173147(v=vs.80).aspx

EDIT:

If you need to do some other logic besides the equality of two DBTileSimple objects, then I think it would be best to create another method that handles this logic. Equality has to remain as a way of comparing the equality of two objects, otherwise it might cause confusion in the future.

IEqualityComparer not working at all

3 Answers3