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

.net, double, epsilon, hashcode, iequalitycomparer

Solution

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));
}

Problem

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...

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