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));
    }
}

Answer 2

Like other non-primitive built-in types, it just returns something arbitrary. It definitely doesn't try to hash the contents of the array. See this answer.

Answer 3

Simple solution

    public static int GetHashFromBytes(byte[] bytes)
    {
        return new BigInteger(bytes).GetHashCode();
    }

Answer 4

byte[] inherits GetHashCode() from object, it doesn't override it. So what you get is basically object's implementation.

Answer 5

If you are using .NET 6 or at least .NET Core 2.1, you can write less codes and achieve better performance with System.HashCode struct.

Using the method HashCode.AddBytes() which available from .NET 6:

public int GetHashCode(byte[] value)
{
    var hash = new HashCode();
    hash.AddBytes(value);
    return hash.ToHashCode();
}

Using the method HashCode.Add which available from .NET Core 2.1:

public int GetHashCode(byte[] value) =>
    value.Aggregate(new HashCode(), (hash, i) => {
        hash.Add(i);
        return hash;
    }).ToHashCode();

Note in the document of HashCode.AddBytes() it says:

This method does not guarantee that the result of adding a span of bytes will match the result of adding the same bytes individually.

In this sharplab demo they are just output same result, but this might be varying from .NET version or runtime environment.

Answer 6

If it's not the same instance, it will return different hashes. I'm guessing it is based on the memory address where it is stored somehow.