IEqualityComparer and weird results

Question

Hello guys!

Take a look at this class:

public class MemorialPoint:IMemorialPoint,IEqualityComparer<MemorialPoint>
{
    private string _PointName;
    private IPoint _PointLocation;
    private MemorialPointType _PointType;

    private DateTime _PointStartTime;
    private DateTime _PointFinishTime;

    private string _NeighborName;

    private double _Rms;
    private double _PointPdop;
    private double _PointHdop;
    private double _PointVdop;

    // getters and setters omitted

    public bool Equals(MemorialPoint x, MemorialPoint y)
    {
        if (x.PointName == y.PointName)
            return true;
        else if (x.PointName == y.PointName && x.PointLocation.X == y.PointLocation.X && x.PointLocation.Y == y.PointLocation.Y)
            return true;
        else
            return false;
    }

    public int GetHashCode(MemorialPoint obj)
    {
        return (obj.PointLocation.X.ToString() + obj.PointLocation.Y.ToString() + obj.PointName).GetHashCode();
    }
}

I also have a Vector class, which is merely two points and some other atributes. I don't want to have equal points in my Vector, so I came up with this method:

public void RecalculateVector(IMemorialPoint fromPoint, IMemorialPoint toPoint, int partIndex)
        {
            if (fromPoint.Equals(toPoint))
                throw new ArgumentException(Messages.VectorWithEqualPoints);

            this.FromPoint = FromPoint;
            this.ToPoint = ToPoint;
            this.PartIndex = partIndex;

            // the constructDifference method has a weird way of working:
            // difference of Point1 and Point 2, so point2 > point1 is the direction
            IVector3D vector = new Vector3DClass();
            vector.ConstructDifference(toPoint.PointLocation, fromPoint.PointLocation);

            this.Azimuth = MathUtilities.RadiansToDegrees(vector.Azimuth);

            IPointCollection pointCollection = new PolylineClass();
            pointCollection.AddPoint(fromPoint.PointLocation, ref _missing, ref _missing);
            pointCollection.AddPoint(toPoint.PointLocation, ref _missing, ref _missing);

            this._ResultingPolyline = pointCollection as IPolyline;
        }

And this unit test, which should give me an exception:

    [TestMethod]
    [ExpectedException(typeof(ArgumentException), Messages.VectorWithEqualPoints)]
    public void TestMemoriaVector_EqualPoints()
    {
        IPoint p1 = PointPolygonBuilder.BuildPoint(0, 0);
        IPoint p2 = PointPolygonBuilder.BuildPoint(0, 0);

        IMemorialPoint mPoint1 = new MemorialPoint("teste1", p1);
        IMemorialPoint mPoint2 = new MemorialPoint("teste1", p2);

        Console.WriteLine(mPoint1.GetHashCode().ToString());
        Console.WriteLine(mPoint2.GetHashCode().ToString());

        vector = new MemorialVector(mPoint1, mPoint1, 0);
    }

When i use the same point, that is, mPoint1, as in the code the exception is thrown. When I use mPoint2, even their name and coordinates being the same, the exception is not thrown. I checked their hash codes, and they are in fact different. Based on the code I created in GetHashCode, I tought these two point would have the same hashcode.

Can someone explain to me why this is not working as I tought it would? I'm not sure I explained this well, but.. I appreciate the help :D

George

Answer 1

You need to override Object.Equals as well.

Add this to your implementation:

// In MemorialPoint:
public override bool Equals(object obj)
{
    if (obj == null || GetType() != obj.GetType()) 
         return false;

    MemorialPoint y = obj as MemorialPoint;

    if (this.PointName == y.PointName)
        return true;
    else if (this.PointName == y.PointName && this.PointLocation.X == y.PointLocation.X && this.PointLocation.Y == y.PointLocation.Y)
        return true;
    else
        return false;
}

I'd then rework your other implementation to use the first, plus add the appropriate null checks.

public bool Equals(MemorialPoint x, MemorialPoint y)
{
    if (x == null)
        return (y == null);
    return x.Equals(y);
}

Answer 2

You're implementing IEqualityComparer<T> within the type it's trying to compare - which is very odd. You should almost certainly just be implementing IEquatable<T> and overriding Equals(object) instead. That would definitely make your unit test work.

The difference between IEquatable<T> and IEqualityComparer<T> is that the former is implemented by a class to say, "I can compare myself with another instance of the same type." (It doesn't have to be the same type, but it usually is.) This is appropriate if there's a natural comparison - for example, the comparison chosen by string is ordinal equality - it's got to be exactly the same sequence of char values.

Now IEqualityComparer<T> is different - it can compare any two instances of a type. There can be multiple different implementations of this for a given type, so it doesn't matter whether or not a particular comparison is "the natural one" - it's just got to be the right one for your job. So for example, you could have a Shape class, and different equality comparers to compare shapes by colour, area or something like that.

Answer 3

You also need to rethink your concept of "equality", since it's not currently meeting .NET framework requirements.

If at all possible, I recommend a re-design with a Repository of memorial point objects (possibly keyed by name), so that simple reference equality can be used.

Answer 4

You've put an arcobjects tag on this, so I just thought I'd mention IRelationalOperator.Equals. I've never tested to see if this method honors the cluster tolerance of the geometries' spatial references. This can be adjusted using ISpatialReferenceTolerance.XYTolerance.