GetHashCode override of object containing generic array
arrays, c#, generics, hashcode
Solution
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);
}
}
Problem
I have a class that contains the following two properties: ``` public int Id { get; private set; } public T[] Values { get; private set; } ``` I have made it `IEquatable<T>` and overriden the `object.Equals` like this: ``` 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); } ``` When having override `object.Equals` I must also override `GetHashCode` of course. But what code should I implement? How do I create a hashcode out of a generic array? And how do I combine it with the `Id` integer? ``` public override int GetHashCode() { return // What? } ```