c# - GetHashCode() on byte[] array

Question

What does GetHashCode() calculate when invoked on the byte[] array? The 2 data arrays with equal content do not provide the same hash.

Answer 1

Arrays in .NET don't override Equals or GetHashCode, so the value you'll get is basically based on reference equality (i.e. the default implementation in Object) - for value equality you'll need to roll your own code (or find some from a third party). You may want to implement IEqualityComparer<byte[]> if you're trying to use byte arrays as keys in a dictionary etc.

EDIT: Here's a reusable array equality comparer which should be fine so long as the array element handles equality appropriately. Note that you mustn't mutate the array after using it as a key in a dictionary, otherwise you won't be able to find it again - even with the same reference.

using System;
using System.Collections.Generic;

public sealed class ArrayEqualityComparer<T> : IEqualityComparer<T[]>
{
    // You could make this a per-instance field with a constructor parameter
    private static readonly EqualityComparer<T> elementComparer
        = EqualityComparer<T>.Default;

    public bool Equals(T[] first, T[] second)
    {
        if (first == second)
        {
            return true;
        }
        if (first == null || second == null)
        {
            return false;
        }
        if (first.Length != second.Length)
        {
            return false;
        }
        for (int i = 0; i < first.Length; i++)
        {
            if (!elementComparer.Equals(first[i], second[i]))
            {
                return false;
            }
        }
        return true;
    }

    public int GetHashCode(T[] array)
    {
        unchecked
        {
            if (array == null)
            {
                return 0;
            }
            int hash = 17;
            foreach (T element in array)
            {
                hash = hash * 31 + elementComparer.GetHashCode(element);
            }
            return hash;
        }
    }
}

class Test
{
    static void Main()
    {
        byte[] x = { 1, 2, 3 };
        byte[] y = { 1, 2, 3 };
        byte[] z = { 4, 5, 6 };

        var comparer = new ArrayEqualityComparer<byte>();

        Console.WriteLine(comparer.GetHashCode(x));
        Console.WriteLine(comparer.GetHashCode(y));
        Console.WriteLine(comparer.GetHashCode(z));
        Console.WriteLine(comparer.Equals(x, y));
        Console.WriteLine(comparer.Equals(x, z));
    }
}