Question

What is the best hash method for an array of byte?

The arrays are serialized class objects containing jpeg image passed between applications over TCP/IP.

The array size is about 200k.

Answer 1

Any of the built-in hashing functions should do; depending on how much you care about collisions these are your options (from most collisions to least):

• MD5
• SHA1
• SHA256
• SHA384
• SHA512

They are as simple to use as:

var hash = SHA1.Create().ComputeHash(data);

Bonus Marks: If you don’t care about security (which I don’t think you do given that you are getting the hashes for images) you might want to look into Murmur hash, which is designed for content hashing and not secure hashing (and is thus much faster). It isn’t, however, in the framework so you will have to find an implementation (and you should probably go for Murmur3).

Edit: If you are looking for a HASHCODE for a byte[] array it’s entirely up to you, it usually consists of bit shifting (by primes) and XORing. E.g.

public class ByteArrayEqualityComparer : IEqualityComparer<byte[]>
{
    public static readonly ByteArrayEqualityComparer Default = new ByteArrayEqualityComparer();
    private ByteArrayEqualityComparer() { }

    public bool Equals(byte[] x, byte[] y)
    {
        if (x == null && y == null)
            return true;
        if (x == null || y == null)
            return false;
        if (x.Length != y.Length)
            return false;
        for (var i = 0; i < x.Length; i++)
            if (x[i] != y[i])
                return false;
        return true;
    }

    public int GetHashCode(byte[] obj)
    {
        if (obj == null || obj.Length == 0)
            return 0;
        var hashCode = 0;
        for (var i = 0; i < obj.Length; i++)
            // Rotate by 3 bits and XOR the new value.
            hashCode = (hashCode << 3) | (hashCode >> (29)) ^ obj[i];
        return hashCode;
    }
}
// ...
var hc = ByteArrayEqualityComparer.Default.GetHashCode(data);

EDIT: If you want to validate that the value hasn’t changed you should use CRC32.