hash.cs

// Adapted from http://stackoverflow.com/questions/263400/what-is-the-best-algorithm-for-an-overridden-system-object-gethashcode
namespace Utilities
{
    public struct HashCode
    {
        private readonly int hashCode;

        private HashCode(int hashCode)
        {
            this.hashCode = hashCode;
        }

        public static HashCode Start
        {
            get { return new HashCode(17); }
        }

        public static implicit operator int(HashCode hashCode)
        {
            return hashCode.GetHashCode();
        }

        public HashCode Hash<T>(T obj)
        {
            var hash = !Equals(obj, default(T)) ? obj.GetHashCode() : 0;
            return new HashCode(Hash(hash));
        }

        public HashCode Hash<T, K>(T obj, Func<T, K> expression)
        {
            int hash = 0;
            if (Equals(obj, default(T)) == false)
            {
                K value = expression.Invoke(obj);

                if (Equals(value, default(K)) == false)
                {
                    hash = expression.Invoke(obj).GetHashCode();
                }
            }

            return new HashCode(Hash(hash));
        }

        public override int GetHashCode()
        {
            return this.hashCode;
        }

        private int Hash(int newHashCode)
        {
            unchecked
            {
                return newHashCode + this.hashCode * 31; // 31 because it is a shift and subtract on the CPU
            } 
        }
    }