Saving Date and Names to IsolatedStorage

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;
using System.Xml.Linq;
using System.Collections.ObjectModel;
using System.IO.IsolatedStorage;
using System.IO;
using System.Xml;
using System.Xml.Serialization;

namespace LasnaoloAppsi
{
    public partial class OppilasLista : PhoneApplicationPage
    {
        XDocument lista = XDocument.Load("NykyisetKurssit.xml");
        XDocument oppilasInfo = XDocument.Load("Oppilaat.xml");
        private ObservableCollection<Kurssit> _Kurssit = new ObservableCollection<Kurssit>();
        List<Kurssit> selected = new List<Kurssit>();
        string id = string.Empty;

        public OppilasLista()
        {
            InitializeComponent();

        }

        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            if (NavigationContext.QueryString.TryGetValue("id", out id))
            {

                var ryhma = (from ryhmaInfo in lista.Descendants("Kurssi")
                             where ryhmaInfo.Attribute("id").Value == id
                             select new Kurssit
                             {
                                 RyhmanNimi = (string)ryhmaInfo.Element("tunnus").Value

                             }).FirstOrDefault();

                PageTitle.Text = ryhma.RyhmanNimi;

                var oppilas = (from oppilaat in oppilasInfo.Descendants("Oppilas")
                            where oppilaat.Attribute("ryhma").Value == id
                            select new Kurssit
                            {
                                OppilaanNimi = (string)oppilaat.Element("nimi").Value
                            });

                foreach (var item in oppilas)
                    _Kurssit.Add(item);

                OppilaidenLista.ItemsSource = _Kurssit;

            }

            base.OnNavigatedTo(e);
        }

        private void Tallenna_Button_Click(object sender, RoutedEventArgs e)
        {
            selected = _Kurssit.Where(x => x.IsChecked == true).ToList();

            const string id1 = "504T11";
            const string id2 = "504T12";
            const string id3 = "504T10";
            const string id4 = "504T09";

            string r504T11 = "People.xml";
            string r504T12 = "People2.xml";
            string r504T10 = "People3.xml";
            string r504T09 = "People4.xml";

            string d504T11 = "CurrentDate504T110";
            string d504T12 = "CurrentDate504T120";
            string d504T10 = "CurrentDate504T100";
            string d504T09 = "CurrentDate504T090";

            switch (id)
            {
                case id1:

                    saveLasnaolot(r504T11);
                    saveLasnaoloDate(d504T11);
                    break;

                case id2:

                    saveLasnaolot(r504T12);
                    saveLasnaoloDate(d504T12);
                    break;

                case id3:
                    saveLasnaolot(r504T10);
                    saveLasnaoloDate(d504T10);
                    break;

                case id4:
                    saveLasnaolot(r504T09);
                    saveLasnaoloDate(d504T09);
                    break;
            }

        }

        public void saveLasnaoloDate(string currentDate)
        {
            int i = 1;
            var settings = IsolatedStorageSettings.ApplicationSettings;

            Kurssit date = new Kurssit()
            {
                Aika = DateTime.Now.ToString("dd/MM/yyyy") + string.Format(" {0}", i.ToString())
            };

            if (settings.Contains(currentDate))
            {
                string findFileNumber = currentDate.Substring(17, 1);
                int foundNum = int.Parse(findFileNumber);
                int newNum = foundNum++;
                string newFilename = currentDate + newNum.ToString();
                settings[newFilename] = date;
            }
            else
                settings.Add(currentDate, date);

            settings.Save();
        }

        public void saveLasnaolot(string fileName)
        {
            // Write to the Isolated Storage
            XmlWriterSettings x_W_Settings = new XmlWriterSettings();
            x_W_Settings.Indent = true;
            using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
            {
                using (IsolatedStorageFileStream stream = ISF.OpenFile(fileName, FileMode.Create))
                {
                    XmlSerializer serializer = new XmlSerializer(typeof(List<Kurssit>));
                    using (XmlWriter xmlWriter = XmlWriter.Create(stream, x_W_Settings))

                        serializer.Serialize(xmlWriter, selected);
                    MessageBox.Show("Läsnäolot Tallennettu Arkistoon!!!! " + DateTime.Now.ToString("dd/MM/yyyy"));


                }
                }
            }
        }
}