Create hash with salt

Hashing.cs

public class Hashing
    {
        private readonly int SALT_SIZE = 16;
        private readonly int HASH_SIZE = 32;

        private byte[] salt;
        private byte[] newSalt;
        public string HashWithSalt(string passwordPlainText, string Username)
        {
            byte[] key;
            CreateSalt();
            salt = newSalt;
            using (var derive = new Rfc2898DeriveBytes(passwordPlainText, salt))
            {
                key = derive.GetBytes(HASH_SIZE);
            }
            passHash = key;

            return Convert.ToBase64String(key);
        }

        private void CreateSalt()
        {
            newSalt = new byte[SALT_SIZE]; 

            RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
            rng.GetBytes(newSalt);
        }

        public string isValid(string password)
        {
            using (var derive = new Rfc2898DeriveBytes(password,salt))
            {
                byte[] newKey = derive.GetBytes(HASH_SIZE);

                if (!newKey.SequenceEqual(passHash))
                    return "INVALID";
            }
            return "VALID";
        }
    }