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.