How Generate auto generation number

private string RandomString(int size)
        {
            StringBuilder builder = new StringBuilder();
            Random random = new Random();
            char ch;
            int size1;
            if (size == 0)
                size1 = 6;
            else size1 = size;
            for (int i = 0; i < size1; i++)
            {
                ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26* random.NextDouble() +97)));
                builder.Append(ch);
            }

            return builder.ToString();
        }