IEqualityComparer giving wrong result

Question

Here is my test code (Contains is not returning true when it should be):

HashSet<TilePosition> test = new HashSet<TilePosition>(new TilePositionCompare());
test.Add(new TilePosition(10,10));

if (test.Contains(new TilePosition(10,10)))
Debug.Log("We should see this");

My Compare Class: using System.Collections.Generic;

public class TilePositionCompare : IEqualityComparer<TilePosition>
{
    public bool Equals(TilePosition tileA, TilePosition tileB)
    {
        return tileA.PosX == tileB.PosX && tileA.PosY == tileB.PosY;
    }

    public int GetHashCode(TilePosition tile)
    {
        return tile.GetHashCode();
    }
}

Here is the class I want to compare:

using System.Collections;
public class TilePosition
{
int posX;
int posY;

public TilePosition (int posX, int posY)
{
    this.posX = posX;
    this.posY = posY;
}
}

I'm not sure where I am going wrong. I've followed some tutorials that appear to do the same thing.

You seem to be using properties PosX and PosY in the Equals method, where are they in your class? — HasaniH, Jan 10 '14 at 23:00

score 4 · Accepted Answer · edited May 23 '17 at 12:03

4

The GetHashCode() should be based upon X/Y.

public class TilePositionCompare : IEqualityComparer<TilePosition>
{
    public bool Equals(TilePosition tileA, TilePosition tileB)
    {
        return tileA.PosX == tileB.PosX && tileA.PosY == tileB.PosY;
    }

    public int GetHashCode(TilePosition tile)
    {
        var hash = 17;
        hash = hash * 23 + tile.PosX.GetHashCode();
        hash = hash * 23 + tile.PosY.GetHashCode();
        return hash;
    }
}

Example adopted from https://stackoverflow.com/a/263416/70386

To know why a odd prime number is used (17), read here: Why does Java's hashCode() in String use 31 as a multiplier?

edited May 23 '17 at 12:03

Community

1
1

answered Jan 10 '14 at 22:52

jgauffin

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More generally, if `Equals(x, y)` is true for any two objects *x* and *y*, then a correct implementation of `GetHashCode` *must* ensure that `GetHashCode(x)==GetHashCode(y)`. Failure to meet this requirement is the true cause of the "wrong result" reported by the original question. – Sam Harwell Jan 10 '14 at 22:59

IEqualityComparer giving wrong result

1 Answers1