Question

Consider the following code:

byte[] bytes = new byte[] { 1, 2, 5, 0, 6 };
byte[] another = new byte[] { 1, 2, 5, 0, 6 };

Hashtable ht = new Hashtable();
ht.Add(bytes, "hi");
Assert.IsTrue(ht.ContainsKey(another));

Why does this assertion fail? Being an array of a primitive type shouldn’t use using the object reference, should it? So why would it return false? Is there anything I can do to make this hashtable work?

Answer 1

Here’s a sample implementation:

  class Program {
    static void Main(string[] args) {
      byte[] bytes = new byte[] { 1, 2, 5, 0, 6 };
      byte[] another = new byte[] { 1, 2, 5, 0, 6 };

      Hashtable ht = new Hashtable(new ByteArrayComparer());
      ht.Add(bytes, "hi");
      System.Diagnostics.Debug.Assert(ht.ContainsKey(another));
    }

    private class ByteArrayComparer : IEqualityComparer {
      public int GetHashCode(object obj) {
        byte[] arr = obj as byte[];
        int hash = 0;
        foreach (byte b in arr) hash ^= b;
        return hash;
      }
      public new bool Equals(object x, object y) {
        byte[] arr1 = x as byte[];
        byte[] arr2 = y as byte[];
        if (arr1.Length != arr2.Length) return false;
        for (int ix = 0; ix < arr1.Length; ++ix)
          if (arr1[ix] != arr2[ix]) return false;
        return true;
      }
    }
  }

You should use a stronger hash if you put thousands of arrays in the hash table. Check this post for an example.