.net - IEqualityComparer<double> with a tolerance; how to implement GetHashCode?

Question

I'm implementing a reusable DoubleEqualityComparer (with a custom tolerance: the "epsilon" constructor parameter) to ease the usage of LINQ with sequences of double. For example:

bool myDoubleFound = doubles.Contains(myDouble, new DoubleEqualityComparer(epsilon: 0.01));

What is the right way to implement GetHashCode? Here's the code:

   public class DoubleEqualityComparer : IEqualityComparer<double>, IEqualityComparer<double?>
    {
        private readonly double epsilon;

        public DoubleEqualityComparer(double epsilon)
        {
            if (epsilon < 0)
            {
                throw new ArgumentException("epsilon can't be negative", "epsilon");
            }

            this.epsilon = epsilon;
        }

        public bool Equals(double x, double y)
        {
            return System.Math.Abs(x - y) < this.epsilon;
        }

        public int GetHashCode(double obj)
        {
            // ?
        }
   }

PS: I can always return the same value (ex: GetHashCode(double obj){ return 0; }) to always force the call to Equals(double, double) method (not very performant, I know), but I remember that this solution causes problems when the comparer is used with a dictionary...

Answer 1

I'm not sure using IEqualityComparer<T> is the way to go. Because compared objects are not equals.

Maybe you should consider using a simple Any clause + an utility method :

private static bool DoublesAreNearlyEquals(double d1, double d2, double epsilon = 0.01D)
{
    return System.Math.Abs(d1 - d2) < this.epsilon;
}

private void foo()
{
    var myDoubles = Getdoubles();
    var doubleToSearch = 42D;
    var result = myDoubles.Any(d=>DoublesAreNearlyEquals(d, doubleToSearch));
}