/// <summary>
        /// Updates the mobile no.
        /// </summary>
        public void UpdateMobileNo()
        {
            Console.WriteLine("please enter the firstName for you want to update");
            string firstName = Console.ReadLine();

            Console.WriteLine("please enter the new MobileNo that you want to update");
            string             newMobileNo = Console.ReadLine();
            List <AddressBook> addressList = FileOperation.ReadJsonFile();

            if (Regex.IsMatch(newMobileNo, "^[0-9]{10}"))
            {
                foreach (AddressBook list in addressList)
                {
                    if (list.FirstName.Equals(firstName))
                    {
                        list.MobileNo = newMobileNo;
                    }
                }
            }
            else
            {
                Console.WriteLine("invalid input");
                return;
            }

            FileOperation.WriteToFile(addressList);
            Console.WriteLine("your mobileNo is updated");
            Console.WriteLine("if you want to see you updated mobileNo .please enter 1 else enter other number");
            int userInput = Convert.ToInt32(Console.ReadLine());

            if (userInput == 1)
            {
                AddressBook book = new AddressBook();
                book.ReadFromFile();
            }
        }
        /// <summary>
        /// Adds the details.
        /// </summary>
        /// <exception cref="Exception">raise a exception if condition getting false</exception>
        public void AddDetails()
        {
            try
            {
                Console.WriteLine("please enter the first Name");
                this.firstName = Console.ReadLine();
                Console.WriteLine("please enter the last Name");
                this.lastName = Console.ReadLine();
                Console.WriteLine("please enter the Mobile number");
                string number = Console.ReadLine();
                this.mobileNumber = string.Empty;
                while (true)
                {
                    if (Regex.IsMatch(number, "^[0-9]+$"))
                    {
                        this.mobileNumber = number;
                        break;
                    }
                    else
                    {
                        Console.WriteLine("mobile number is invalid");
                    }
                }

                Console.WriteLine("please enter the Address");
                this.address = Console.ReadLine();
                Console.WriteLine("please enter the city");
                this.city = Console.ReadLine();
                Console.WriteLine("please enter the state");
                this.state = Console.ReadLine();
                Console.WriteLine("please enter the ZipCode");
                string code = Console.ReadLine();
                this.zipCode = string.Empty;
                while (true)
                {
                    if (Regex.IsMatch(code, "^[0-9]+$"))
                    {
                        this.zipCode = code;
                        break;
                    }
                    else
                    {
                        Console.WriteLine("invalid zipCode");
                    }
                }

                ////storing all the details in Address book object through Address constructor.
                AddressBook book = new AddressBook(this.firstName, this.lastName, this.mobileNumber, this.address, this.city, this.state, this.zipCode);

                ////reading Address book data from the json file
                List <AddressBook> list = FileOperation.ReadJsonFile();

                ////adding the AddressBook object to the list
                list.Add(book);

                ////writing the list to the json file.
                FileOperation.WriteToFile(list);
                Console.WriteLine("data added successfully");
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message);
            }
        }