Add Contact Button (Address Book)

/*
 * Created by SharpDevelop.
 * User: Admin
 * Date: 26.5.2021 г.
 * Time: 10:10
 *
 * To change this template use Tools | Options | Coding | Edit Standard Headers.
 */
using System;
using System.Drawing;
using System.Windows.Forms;
using System.Text.RegularExpressions;
using System.IO;
using System.Linq;

namespace addressBook_11a
{
    /// <summary>
    /// Description of AddContact.
    /// </summary>
    public partial class AddContact : Form
    {
        public AddContact()
        {
            //
            // The InitializeComponent() call is required for Windows Forms designer support.
            //
            InitializeComponent();

            //
            // TODO: Add constructor code after the InitializeComponent() call.
            //
        }
        void ButtonAddClick(object sender, EventArgs e)
        {

            string name=textBoxName.Text;
            string phone=textBoxPhone.Text;
            string email=textBoxEmail.Text;

            if (name=="")
            {
                MessageBox.Show("Въведи име на контакт");
                return;
            }


            string phoneString=@"^(0|\+359)[0-9]{9}$";
            Regex rPhone=new Regex(phoneString);
            if (phone != "" && !rPhone.IsMatch(phone))
            {
                MessageBox.Show("Въведете валиден телефон");
                return;
            }


            string emailString=@"^[a-z0-9_ \.]+@[a-z0-9_ \.]+\.[a-z]{2,}$";
            Regex rEmail=new Regex(emailString);
            if (email != "" && !rEmail.IsMatch(email))
            {
                MessageBox.Show("Въведете валиден e-mail");
                return;
            }

            var contact = new Contact(name, phone, email);


            //check for the file
            if (!File.Exists("contacts.csv"))
            {
                var f = File.Create("contacts.csv");
                f.Close();
            }

            //add contact to a file
            var allContacts=File.ReadAllLines("contacts.csv");
            if (allContacts.Any(c => c.Contains(name)))
            {
                var userAnswer = MessageBox
                    .Show("Вече съществува контакт с това има. Искате ли да го добавите като нов?",
                    "Внимание !", MessageBoxButtons.YesNo, MessageBoxIcon.Warning);

                if (userAnswer==DialogResult.No)
                {
                    return;
                }
            }

            File.AppendAllText("contacts.csv", contact.Print());

            MessageBox.Show("Контакта е добавен");
            textBoxName.Text="";
            textBoxPhone.Text="";
            textBoxEmail.Text="";

        }
    }
}