GetHashCode override of object containing generic array
Because of the problems raised in this thread, I'm posting another reply showing what happens if you get it wrong... mainly, that you can't use the array's GetHashCode(); the correct behaviour is that no warnings are printed when you run it... switch the comments to fix it:
using System;using System.Collections.Generic;using System.Linq;static class Program{ static void Main() { // first and second are logically equivalent SimpleTableRow<int> first = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6), second = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6); if (first.Equals(second) && first.GetHashCode() != second.GetHashCode()) { // proven Equals, but GetHashCode() disagrees Console.WriteLine("We have a problem"); } HashSet<SimpleTableRow<int>> set = new HashSet<SimpleTableRow<int>>(); set.Add(first); set.Add(second); // which confuses anything that uses hash algorithms if (set.Count != 1) Console.WriteLine("Yup, very bad indeed"); }}class SimpleTableRow<T> : IEquatable<SimpleTableRow<T>>{ public SimpleTableRow(int id, params T[] values) { this.Id = id; this.Values = values; } public int Id { get; private set; } public T[] Values { get; private set; } public override int GetHashCode() // wrong { return Id.GetHashCode() ^ Values.GetHashCode(); } /* public override int GetHashCode() // right { int hash = Id; if (Values != null) { hash = (hash * 17) + Values.Length; foreach (T t in Values) { hash *= 17; if (t != null) hash = hash + t.GetHashCode(); } } return hash; } */ public override bool Equals(object obj) { return Equals(obj as SimpleTableRow<T>); } public bool Equals(SimpleTableRow<T> other) { // Check for null if (ReferenceEquals(other, null)) return false; // Check for same reference if (ReferenceEquals(this, other)) return true; // Check for same Id and same Values return Id == other.Id && Values.SequenceEqual(other.Values); }}
FWIW, it's very dangerous to use the contents of the Values in your hash code. You should only do this if you can guarantee that it will never change. However, since it is exposed, I don't think guaranteeing it is possible. The hashcode of an object should never change. Otherwise, it loses its value as a key in a Hashtable or Dictionary. Consider the hard-to-find bug of using an object as a key in a Hashtable, its hashcode changes because of an outside influence and you can no longer find it in the Hashtable!