MainPage.xaml.cs

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 Microsoft.Phone.Shell;
using System.Collections.ObjectModel;

namespace MentalStack
{
    public partial class MainPage : PhoneApplicationPage
    {
        private ObservableCollection<Note> thoughts;
        public ObservableCollection<Note> Thoughts{
        get
        {
            if (thoughts == null)
            {
                thoughts = new ObservableCollection<Note>();
            }
            return thoughts;
        }
    }
        // Constructor
        public MainPage()
        {
            InitializeComponent();
            Note item = new Note("hi", "just testing");
            thoughts = new ObservableCollection<Note>();
            thoughts.Add(new Note("aaa", "BBB"));
        }

        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);
            if (PhoneApplicationService.Current.State.ContainsKey("Thoughts"))
            {
                thoughts = (ObservableCollection<Note>)PhoneApplicationService.Current.State["Thoughts"];
            }
        }

        protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
        {
            PhoneApplicationService.Current.State["Thoughts"] = thoughts;
            base.OnNavigatedFrom(e);
        }

        private void addThought(object sender, EventArgs e)
        {
            NavigationService.Navigate(new Uri("/PageAddThought.xaml", UriKind.Relative));
        }
    }
}