CODEWITHSUNDEEP
hashresharperhash-code-uniqueness
programmer
programmer

Reputation: 4392

Why is '397' used for ReSharper GetHashCode override?

Like many of you, I use ReSharper to speed up the development process. When you use it to override the equality members of a class, the code-gen it produces for GetHashCode() looks like:

    public override int GetHashCode()
    {
        unchecked
        {
            int result = (Key != null ? Key.GetHashCode() : 0);
            result = (result * 397) ^ (EditableProperty != null ? EditableProperty.GetHashCode() : 0);
            result = (result * 397) ^ ObjectId;
            return result;
        }
    }

Of course I have some of my own members in there, but what I am wanting to know is why 397?

Upvotes: 169

Views: 28100

Answers (2)

kybernetikos
kybernetikos

Reputation: 8689

The hash that resharper uses looks like a variant of the FNV hash. FNV is frequently implemented with different primes. There's a discussion on the appropriate choice of primes for FNV here.

Upvotes: 19

Nick Johnson
Nick Johnson

Reputation: 101139

Probably because 397 is a prime of sufficient size to cause the result variable to overflow and mix the bits of the hash somewhat, providing a better distribution of hash codes. There's nothing particularly special about 397 that distinguishes it from other primes of the same magnitude.

Upvotes: 184

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