> Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace NotebookApp
{
    internal static class Program
    {
        /// <summary>
        /// Главная точка входа для приложения.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MenuForm());
        }
    }
}

> MenuForm.cs
using NotebookApp.Views.Diary;
using NotebookApp.Views.FriendsList;
using NotebookApp.Views.Reminders;
using System;
using System.Windows.Forms;

namespace NotebookApp
{
    public partial class MenuForm : Form
    {
        public MenuForm()
        {
            InitializeComponent();
        }
        private void ToFriendsListBtn_Click(object sender, EventArgs e)
        {
            FriendListForm friendsListForm = new FriendListForm();
            friendsListForm.Show();
        }
        private void ToDiaryBtn_Click(object sender, EventArgs e)
        {
            DiaryForm diaryForm = new DiaryForm();
            diaryForm.Show();
        }
        private void ToRemindersBtn_Click(object sender, EventArgs e)
        {
            RemindersForm remindersForm = new RemindersForm();
            remindersForm.Show();
        }
    }
}

> DiaryForm.cs

using NotebookApp.Models;
using NotebookApp.Services;
using System;
using System.Collections.Generic;
using System.Data;
using System.Globalization;
using System.Linq;
using System.Windows.Forms;

namespace NotebookApp.Views.Diary
{
    public partial class DiaryForm : Form
    {
        readonly DiaryModel _model;
        public DiaryForm()
        {
            InitializeComponent();
            _model = new DiaryModel();
            UpdateForm();
        }
        public void UpdateForm()
        {
            List<DiaryEntry> diary = _model.DiaryEntriesList;
            LoadListView(diary);
        }
        private void LoadListView(List<DiaryEntry> listToShow)
        {
            DiaryDataGrid.Rows.Clear();
            var orderedList = listToShow.OrderBy(x => x.EntryDate);
            foreach (var item in listToShow)
            {
                string[] row = { DateTimeParser.ToStringWithFormat(item.EntryDate.Date), item.Content };
                DiaryDataGrid.Rows.Add(row);
            }
        }
        private void DeleteBtn_Click(object sender, EventArgs e)
        {
            if (DiaryDataGrid.Rows != null)
            {
                DialogResult responce = MessageBox.Show("Вы правда хотите удалить эту запись?", "Подтверждение 
удаления", MessageBoxButtons.YesNo);
                if (responce == DialogResult.Yes)
                {
                    var selectedItems = DiaryDataGrid.SelectedRows;
                    _model.DeleteEntries(selectedItems);
                }
                List<DiaryEntry> updatedList = _model.DiaryEntriesList;
                LoadListView(updatedList);
            }
        }
        private void FindBtn_Click(object sender, EventArgs e)
        {
            string queredParameter = QueredParameterCmbBx.Text;
            string query = SearchBox.Text;
            List<DiaryEntry> queredList = _model.FindEntry(queredParameter, query);
            LoadListView(queredList);
        }
        private void CreateBtn_Click(object sender, EventArgs e)
        {
            CreateUpdateDiaryEntryForm createForm = new CreateUpdateDiaryEntryForm(this, _model);
            createForm.Show();
        }
        private void UpdateBtn_Click(object sender, EventArgs e)
        {
            var selectedItem = DiaryDataGrid.SelectedRows[0];
            CreateUpdateDiaryEntryForm createForm = new CreateUpdateDiaryEntryForm(this, _model, selectedItem);
            createForm.Show();
        }
    }
}

> CreateUpdateDiaryEntryForm.cs

using NotebookApp.Models;
using System;
using System.Windows.Forms;

namespace NotebookApp.Views.Diary
{
    public partial class CreateUpdateDiaryEntryForm : Form
    {
        readonly DiaryModel _model;
        readonly DiaryForm _parentForm;
        readonly DiaryEntry _oldEntry;
        public CreateUpdateDiaryEntryForm(DiaryForm parentForm, DiaryModel model, DataGridViewRow oldEntry = 
default)
        {
            InitializeComponent();
            _parentForm = parentForm;
            _model = model;
            if (oldEntry != default)
            {
                _oldEntry = new DiaryEntry(oldEntry);
                EntryDatePicker.Value = _oldEntry.EntryDate;
                ContentTextBox.Text = _oldEntry.Content;
            }
        }
        private void CreateUpdateDiaryEntryBtn_Click(object sender, EventArgs e)
        {
            DiaryEntry newEntry = new DiaryEntry(
                entryDate: EntryDatePicker.Value,
                content: ContentTextBox.Text
                );
            if (_oldEntry == default)
                _model.CreateEntry(newEntry);
            else
            {
                _model.UpdateEntry(_oldEntry, newEntry);
            }
            _parentForm.UpdateForm();
            Close();
        }
    }
}

> FriendListForm.cs

using NotebookApp.Models;
using NotebookApp.Services;
using System;
using System.Collections.Generic;
using System.Data;
using System.Globalization;
using System.Linq;
using System.Windows.Forms;

namespace NotebookApp.Views.FriendsList
{
    public partial class FriendListForm : Form
    {
        readonly FriendListModel _model;
        public FriendListForm()
        {
            InitializeComponent();
            _model = new FriendListModel();
            UpdateForm();
        }
        public void UpdateForm()
        {
            List<Friend> friendList = _model.FriendsList;
            LoadListView(friendList);
        }
        private void LoadListView(List<Friend> listToShow)
        {
            FriendsListView.Items.Clear();
            var orderedList = listToShow.OrderBy(x => x.FullName);
            foreach (var item in listToShow)
            {
                string[] row = { item.FullName, DateTimeParser.ToStringWithFormat(item.BirthDate, "d"), 
item.Address, item.PhoneNumber, item.Remark };
                var listItem = new ListViewItem(row);
                FriendsListView.Items.Add(listItem);
            }
        }
        private void DeleteBtn_Click(object sender, EventArgs e)
        {
            if (FriendsListView.SelectedItems != null)
            {
                DialogResult responce = MessageBox.Show("Вы правда хотите удалить эту запись?", "Подтверждение 
удаления", MessageBoxButtons.YesNo);
                if (responce == DialogResult.Yes)
                {
                    var selectedItems = FriendsListView.SelectedItems;
                    _model.DeleteEntries(selectedItems);
                }
                List<Friend> updatedList = _model.FriendsList;
                LoadListView(updatedList);
            }
        }
        private void FindBtn_Click(object sender, EventArgs e)
        {
            string queredParameter = QueredParameterCmbBx.Text;
            string query = SearchBox.Text;
            List<Friend> queredList = _model.FindEntry(queredParameter, query);
            LoadListView(queredList);
        }
        private void CreateBtn_Click(object sender, EventArgs e)
        {
            CreateUpdateFriendForm createForm = new CreateUpdateFriendForm(this, _model);
            createForm.Show();
        }
        private void UpdateBtn_Click(object sender, EventArgs e)
        {
            ListViewItem selectedItem = FriendsListView.SelectedItems[0];
            CreateUpdateFriendForm createForm = new CreateUpdateFriendForm(this, _model, selectedItem);
            createForm.Show();
        }
        private void ShowUpcomingBirthDateCount()
        {
            MessageBox.Show($"Количество ближайших дней рождений: {_model.UpcomingBirthDateCount}", 
"Предупреждение", MessageBoxButtons.OK);
        }
        private void FriendListForm_Load(object sender, EventArgs e)
        {
            if (_model.UpcomingBirthDateCount != 0) ShowUpcomingBirthDateCount();
        }
    }
}

6.	CreateUpdateFriendForm.cs

using NotebookApp.Models;
using System;
using System.Windows.Forms;

namespace NotebookApp.Views.FriendsList
{
    public partial class CreateUpdateFriendForm : Form
    {
        readonly FriendListModel _model;
        readonly FriendListForm _parentForm;
        readonly Friend _oldFriend;
        public CreateUpdateFriendForm(FriendListForm parentForm, FriendListModel model, ListViewItem oldEntry = 
default)
        {
            InitializeComponent();
            _parentForm = parentForm;
            _model = model;
            if (oldEntry != default)
            {
                _oldFriend = new Friend(oldEntry);
                FullNameTextBox.Text = _oldFriend.FullName;
                BirthDatePicker.Value = _oldFriend.BirthDate;
                AddressTextBox.Text = _oldFriend.Address;
                PhoneTextBox.Text = _oldFriend.PhoneNumber;
                RemarkTextBox.Text = _oldFriend.Remark;
            }
        }
        private void CreateUpdateFriendBtn_Click(object sender, EventArgs e)
        {
            Friend friend = new Friend(fullName: FullNameTextBox.Text,
                                       birthDate: BirthDatePicker.Value,
                                       address: AddressTextBox.Text,
                                       phoneNumber: PhoneTextBox.Text,
                                       remark: RemarkTextBox.Text);
            if (_oldFriend == default)
                _model.CreateEntry(friend);
            else
            {
                _model.UpdateEntry(_oldFriend, friend);
            }
            _parentForm.UpdateForm();
            Close();
        }
    }
}

7.	RemindersForm.cs

using NotebookApp.Models;
using NotebookApp.Services;
using System;
using System.Collections.Generic;
using System.Data;
using System.Globalization;
using System.Linq;
using System.Windows.Forms;

namespace NotebookApp.Views.Reminders
{
    public partial class RemindersForm : Form
    {
        readonly RemindersModel _model;
        public RemindersForm()
        {
            InitializeComponent();
            _model = new RemindersModel();
            UpdateForm();
        }
        public void UpdateForm()
        {
            List<Reminder> remindersList = _model.RemindersList;
            LoadListView(remindersList);
        }
        private void LoadListView(List<Reminder> listToShow)
        {
            RemindersListView.Items.Clear();
            var orderedList = listToShow.OrderBy(x => x.EventDate);
            foreach (var item in listToShow)
            {
                string[] row = { DateTimeParser.ToStringWithFormat(item.EventDate), item.Description };
                var listItem = new ListViewItem(row);
                RemindersListView.Items.Add(listItem);
            }
        }
        private void DeleteBtn_Click(object sender, EventArgs e)
        {
            if (RemindersListView.SelectedItems != null)
            {
                DialogResult responce = MessageBox.Show("Вы правда хотите удалить эту запись?", "Подтверждение 
удаления", MessageBoxButtons.YesNo);
                if (responce == DialogResult.Yes)
                {
                    var selectedItems = RemindersListView.SelectedItems;
                    _model.DeleteEntries(selectedItems);
                }
                List<Reminder> updatedList = _model.RemindersList;
                LoadListView(updatedList);
            }
        }
        private void FindBtn_Click(object sender, EventArgs e)
        {
            string queredParameter = QueredParameterCmbBx.Text;
            string query = SearchBox.Text;
            List<Reminder> queredList = _model.FindEntry(queredParameter, query);
            LoadListView(queredList);
        }
        private void CreateBtn_Click(object sender, EventArgs e)
        {
            CreateUpdateRemindersForm createForm = new CreateUpdateRemindersForm(this, _model);
            createForm.Show();
        }
        private void UpdateBtn_Click(object sender, EventArgs e)
        {
            ListViewItem selectedItem = RemindersListView.SelectedItems[0];
            CreateUpdateRemindersForm createForm = new CreateUpdateRemindersForm(this, _model, selectedItem);
            createForm.Show();
        }
        private void ShowUpcomingEventsCount()
        {
            MessageBox.Show($"Количество ближайших событий: {_model.UpcomingEventsCount}", "Предупреждение", 
MessageBoxButtons.OK);
        }
        private void RemindersForm_Load(object sender, EventArgs e)
        {
            if (_model.UpcomingEventsCount != 0) ShowUpcomingEventsCount();
        }
    }
}

8.	CreateUpdateRemindersForm.cs

using NotebookApp.Models;
using System;
using System.Windows.Forms;

namespace NotebookApp.Views.Reminders
{
    public partial class CreateUpdateRemindersForm : Form
    {
        readonly RemindersModel _model;
        readonly RemindersForm _parentForm;
        readonly Reminder _oldEntry;
        public CreateUpdateRemindersForm(RemindersForm parentForm, RemindersModel model, ListViewItem oldEntry = 
default)
        {
            InitializeComponent();
            _parentForm = parentForm;
            _model = model;
            if (oldEntry != default)
            {
                _oldEntry = new Reminder(oldEntry);
                EventDatePicker.Value = _oldEntry.EventDate;
                EventDescriptionTextBox.Text = _oldEntry.Description;
            }
        }
        private void CreateUpdateReminderBtn_Click(object sender, EventArgs e)
        {
            Reminder newEntry = new Reminder(
                eventDate: EventDatePicker.Value,
                description: EventDescriptionTextBox.Text
                );
            if (_oldEntry == default)
                _model.CreateEntry(newEntry);
            else
            {
                _model.UpdateEntry(_oldEntry, newEntry);
            }
            _parentForm.UpdateForm();
            Close();
        }
    }
}

9.	ICrudModel.cs

using System.Collections;
using System.Collections.Generic;

namespace NotebookApp.Interfaces
{
    internal interface ICrudModel<T>
    {
        void CreateEntry(T newEntry);
        void UpdateEntry(T oldEntry, T modifiedEntry);
        void DeleteEntries(ICollection selectedEntries);
        List<T> FindEntry(string queredParameter, string query);
    }
}

10.	DiaryEntry.cs

using NotebookApp.Services;
using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace NotebookApp.Models
{
    public class DiaryEntry
    {
        readonly DateTime _entryDate;
        readonly string _content;
        public DateTime EntryDate => _entryDate;
        public string Content => _content;
        public DiaryEntry()
        {
        }
        public DiaryEntry(DateTime entryDate, string content)
        {
            _entryDate = entryDate;
            _content = content;
        }
        public DiaryEntry(DataGridViewRow listViewItem) : this(DataGridItemToString(listViewItem)) { }
        public DiaryEntry(string rawData)
        {
            string[] splitData = rawData.Split(';');
            if (!DateTimeParser.TryParse(splitData[0], out _entryDate))
                _entryDate = DateTime.Now;
            _content = splitData[1];
        }
        public override string ToString()
        {
            return string.Join(";", EntryDate.ToString(), Content);
        }
        public override bool Equals(object obj)
        {
            try
            {
                DiaryEntry diaryEntry = (DiaryEntry)obj;
                return diaryEntry.EntryDate.Date == EntryDate.Date
                    && diaryEntry.Content == Content;
            }
            catch
            {
                return false;
            }
        }
        public override int GetHashCode()
        {
            return base.GetHashCode();
        }
        private static string DataGridItemToString(DataGridViewRow item)
        {
            List<string> unpackedListViewItem = new List<string>();
            foreach (DataGridViewCell subItem in item.Cells)
            {
                unpackedListViewItem.Add((string)subItem.Value);
            }
            return string.Join(";", unpackedListViewItem);
        }
    }
}

11.	DiaryModel.cs

using NotebookApp.Interfaces;
using NotebookApp.Services;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Windows.Forms;

namespace NotebookApp.Models
{
    public class DiaryModel : ICrudModel<DiaryEntry>
    {
        readonly FileService<DiaryEntry> _fileService;
        readonly List<DiaryEntry> _diaryEntriesList;
        readonly string _fileName = "diary.txt";
        public List<DiaryEntry> DiaryEntriesList => _diaryEntriesList;
        public DiaryModel()
        {
            _fileService = new FileService<DiaryEntry>();
            _diaryEntriesList = _fileService.ReadFromFile(_fileName);
        }
        public void CreateEntry(DiaryEntry newEntry)
        {
            _diaryEntriesList.Add(newEntry);
            _fileService.WriteToFile(_diaryEntriesList, _fileName);
        }
        public void DeleteEntries(ICollection selectedEntries)
        {
            foreach (DataGridViewRow item in selectedEntries)
            {
                DiaryEntry entryToRemove = new DiaryEntry(item);
                _diaryEntriesList.Remove(entryToRemove);
                _fileService.WriteToFile(_diaryEntriesList, _fileName);
            }
        }
        public List<DiaryEntry> FindEntry(string queredParameter, string query)
        {
            switch (queredParameter)
            {
                case "Дата":
                    return _diaryEntriesList.Where(x => x.EntryDate.Date == DateTime.Parse(query, 
CultureInfo.GetCultureInfo("ru-RU"))).ToList();
                case "Запись":
                    return _diaryEntriesList.Where(x => x.Content.Contains(query)).ToList();
                default:
                    return _diaryEntriesList;
            }
        }
        public void UpdateEntry(DiaryEntry oldEntry, DiaryEntry modifiedEntry)
        {
            int entryIndex = _diaryEntriesList.IndexOf(oldEntry);
            _diaryEntriesList[entryIndex] = modifiedEntry;
            _fileService.WriteToFile(_diaryEntriesList, _fileName);
        }
    }
}

12.	Friend.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Windows.Forms;

namespace NotebookApp.Models
{
    public class Friend
    {
        readonly string _fullName;
        readonly DateTime _birthDate;
        readonly string _address;
        readonly string _phoneNumber;
        readonly string _remark;
        public string FullName => _fullName;
        public DateTime BirthDate => _birthDate;
        public string Address => _address;
        public string PhoneNumber => _phoneNumber;
        public string Remark => _remark;
        public Friend(string fullName, DateTime birthDate, string address, string phoneNumber, string remark)
        {
            _fullName = fullName;
            _birthDate = birthDate;
            _address = address;
            _phoneNumber = phoneNumber;
            _remark = remark;
        }
        public Friend(ListViewItem listViewItem) : this(ListViewItemToString(listViewItem)) { }
        public Friend(string rawData)
        {
            string[] splitData = rawData.Split(';');
            string[] dateFormats = { "dd.MM.yyyy", "G" };
            _fullName = splitData[0];
            if (!DateTime.TryParseExact(splitData[1], dateFormats, null, DateTimeStyles.None, out _birthDate))
                _birthDate = DateTime.Now;
            _address = splitData[2];
            _phoneNumber = splitData[3];
            _remark = splitData[4];
        }
        public Friend() { }
        public override string ToString()
        {
            return string.Join(";", _fullName, _birthDate.ToString(), _address, _phoneNumber, _remark);
        }
        public override bool Equals(object obj)
        {
            try
            {
                Friend friend = (Friend)obj;
                return friend.FullName == FullName
                    && friend.BirthDate.Date == BirthDate.Date
                    && friend.Address == Address
                    && friend.PhoneNumber == PhoneNumber
                    && friend.Remark == Remark;
            }
            catch
            {
                return false;
            }
        }
        public override int GetHashCode()
        {
            return base.GetHashCode();
        }
        private static string ListViewItemToString(ListViewItem item)
        {
            List<string> unpackedListViewItem = new List<string>();
            foreach (ListViewItem.ListViewSubItem subItem in item.SubItems)
            {
                unpackedListViewItem.Add(subItem.Text);
            }
            return string.Join(";", unpackedListViewItem);
        }
    }
}

13.	FriendListModel.cs

using NotebookApp.Interfaces;
using NotebookApp.Services;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

namespace NotebookApp.Models
{
    public class FriendListModel : ICrudModel<Friend>
    {
        readonly FileService<Friend> _fileService;
        readonly List<Friend> _friendsList;
        readonly string _fileName = "friends.txt";
        int _upcomingBirthDateCount;
        public int UpcomingBirthDateCount => _upcomingBirthDateCount;
        public List<Friend> FriendsList => _friendsList;
        
        public FriendListModel()
        {
            _fileService = new FileService<Friend>();
            _friendsList = _fileService.ReadFromFile(_fileName);
            CalculateUpcomingBirthDateCount();
        }
        public void CreateEntry(Friend newEntry)
        {
            _friendsList.Add(newEntry);
            _fileService.WriteToFile(_friendsList, _fileName);
        }
        public void UpdateEntry(Friend oldEntry, Friend modifiedEntry)
        {
            int entryIndex = _friendsList.IndexOf(oldEntry);
            _friendsList[entryIndex] = modifiedEntry;
            _fileService.WriteToFile(_friendsList, _fileName);
        }
        public List<Friend> FindEntry(string queredParameter, string query)
        {
            switch (queredParameter)
            {
                case "ФИО":
                    return _friendsList.Where(x => x.FullName.Contains(query)).ToList();
                case "Телефон":
                    return new List<Friend> { _friendsList.First(x => x.PhoneNumber.Contains(query)) };
                case "Адрес":
                    return _friendsList.Where(x => x.Address.Contains(query)).ToList();
                default:
                    return _friendsList;
            }
        }
        public void DeleteEntries(ICollection selectedEntries)
        {
            foreach (ListViewItem item in selectedEntries)
            {
                Friend friendToRemove = new Friend(item);
                _friendsList.Remove(friendToRemove);
                _fileService.WriteToFile(_friendsList, _fileName);
            }
        }
        private void CalculateUpcomingBirthDateCount() 
        {
            foreach (Friend eventEntry in FriendsList)
            {
                TimeSpan dateDiff = eventEntry.BirthDate.Date - DateTime.Now.Date;
                if (dateDiff > TimeSpan.Zero && dateDiff < TimeSpan.FromDays(5))
                {
                    _upcomingBirthDateCount++;
                }
            }
        }
    }
}

14.	Reminder.cs

using NotebookApp.Services;
using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace NotebookApp.Models
{
    public class Reminder
    {
        readonly DateTime _eventDate;
        readonly string _description;
        public Reminder()
        {
        }
        public Reminder(DateTime eventDate, string description)
        {
            _eventDate = eventDate;
            _description = description;
        }
        public DateTime EventDate => _eventDate;
        public string Description => _description;
        public Reminder(ListViewItem listViewItem) : this(ListViewItemToString(listViewItem)) { }
        public Reminder(string rawData)
        {
            string[] splitData = rawData.Split(';');
            if (!DateTimeParser.TryParse(splitData[0], out _eventDate))
                _eventDate = DateTime.Now;
            _description = splitData[1];
        }
        public override string ToString()
        {
            return string.Join(";", EventDate.ToString(), Description);
        }
        public override bool Equals(object obj)
        {
            try
            {
                Reminder reminder = (Reminder)obj;
                return reminder.EventDate == EventDate
                    && reminder.Description == Description;
            }
            catch
            {
                return false;
            }
        }
        public override int GetHashCode()
        {
            return base.GetHashCode();
        }
        private static string ListViewItemToString(ListViewItem item)
        {
            List<string> unpackedListViewItem = new List<string>();
            foreach (ListViewItem.ListViewSubItem subItem in item.SubItems)
            {
                unpackedListViewItem.Add(subItem.Text);
            }
            return string.Join(";", unpackedListViewItem);
        }
    }
}

15.	RemindersModel.cs

using NotebookApp.Interfaces;
using NotebookApp.Services;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Windows.Forms;

namespace NotebookApp.Models
{
    public class RemindersModel : ICrudModel<Reminder>
    {
        readonly FileService<Reminder> _fileService;
        readonly List<Reminder> _remindersList;
        readonly string _fileName = "reminders.txt";
        int _upcomingEventsCount;
        public List<Reminder> RemindersList => _remindersList;
        public int UpcomingEventsCount => _upcomingEventsCount;
        public RemindersModel()
        {
            _fileService = new FileService<Reminder>();
            _remindersList = _fileService.ReadFromFile(_fileName);
            CalculateUpcomingEventsCount();
        }
        public void CreateEntry(Reminder newEntry)
        {
            _remindersList.Add(newEntry);
            _fileService.WriteToFile(_remindersList, _fileName);
        }
        public void DeleteEntries(ICollection selectedEntries)
        {
            foreach (ListViewItem item in selectedEntries)
            {
                Reminder entryToRemove = new Reminder(item);
                _remindersList.Remove(entryToRemove);
                _fileService.WriteToFile(_remindersList, _fileName);
            }
        }
        public List<Reminder> FindEntry(string queredParameter, string query)
        {
            switch (queredParameter)
            {
                case "Дата":
                    return _remindersList.Where(x => x.EventDate.Date == DateTime.Parse(query, 
CultureInfo.GetCultureInfo("ru-RU"))).ToList();
                case "Событие":
                    return _remindersList.Where(x => x.Description.Contains(query)).ToList();
                default:
                    return _remindersList;
            }
        }
        public void UpdateEntry(Reminder oldEntry, Reminder modifiedEntry)
        {
            int entryIndex = _remindersList.IndexOf(oldEntry);
            _remindersList[entryIndex] = modifiedEntry;
            _fileService.WriteToFile(_remindersList, _fileName);
        }
        private void CalculateUpcomingEventsCount()
        {
            foreach (Reminder eventEntry in RemindersList)
            {
                TimeSpan dateDiff = eventEntry.EventDate.Date - DateTime.Now.Date;
                if (dateDiff > TimeSpan.Zero && dateDiff < TimeSpan.FromDays(5))
                {
                    _upcomingEventsCount++;
                }
            }
        }
    }
}

16.	DateTimeParser.cs

using System;
using System.Globalization;

namespace NotebookApp.Services
{
    public static class DateTimeParser
    {
        static readonly string[] dateTimeFormats = { "dd.MM.yyyy", "dd.MM.yyyy H:mm:ss", "G", "M/d/yyyy h:mm:ss 
tt" };
        public static bool TryParse(
            string s,
            out DateTime result,
            DateTimeStyles style = DateTimeStyles.None)
        {
            return DateTime.TryParseExact(s, dateTimeFormats, CultureInfo.InvariantCulture, style, out result);
        }
        public static string ToStringWithFormat(DateTime dt, string format = "G")
        {
            return dt.ToString(format, CultureInfo.GetCultureInfo("ru-RU"));
        }
    }
}

17.	FileService.cs

using System;
using System.Collections.Generic;
using System.IO;

namespace NotebookApp.Services
{
    public class FileService<T> where T : new()
    {
        readonly string _workingDirectory;
        public FileService(string workingDirectory = "../../io")
        {
            if (!Directory.Exists(workingDirectory))
            {
                Directory.CreateDirectory(workingDirectory);
            }
            _workingDirectory = workingDirectory;
        }
        public List<T> ReadFromFile(string fileName)
        {
            List<T> fileInfo = new List<T>();
            string fullPath = Path.Combine(_workingDirectory, fileName);
            if (File.Exists(fullPath))
            {
                using (StreamReader sr = new StreamReader(fullPath))
                {
                    while (!sr.EndOfStream)
                    {
                        var line = sr.ReadLine();
                        if (!string.IsNullOrEmpty(line))
                        {
                            T anotherPieceOfInfo = (T)Activator.CreateInstance(typeof(T), args: line);
                            fileInfo.Add(anotherPieceOfInfo);
                        }
                    }
                }
            }
            return fileInfo;
        }
        public void WriteToFile(List<T> info, string fileName)
        {
            string fullPath = Path.Combine(_workingDirectory, fileName);
            using (StreamWriter sw = new StreamWriter(fullPath))
            {
                foreach (T item in info)
                {
                    sw.WriteLine(item.ToString());
                }
            }
        }
    }
}