Test case for sort issue in DataGrid

using System;
using System.Windows;
using System.Windows.Controls;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Windows.Input;

namespace DataGridTest5
{
    /// <summary>
    /// Test case for sort issue in DataGrid
    /// a) Viewing:
    /// - start the application, the first level is shown automatically
    ///   -> the sort order is OK
    /// - select level "Second"
    /// - select level "First"
    ///   -> the sort order is not correct anymore
    /// b) Editing:
    /// - start the application, the first level is shown automatically
    ///   -> the sort order is OK
    /// - edit names
    ///   -> the DataGrid elements are sorted after each modification
    /// - select level "Second"
    /// - select level "First"
    ///   -> the sort order is not correct anymore
    /// - edit names
    ///   -> the DataGrid elements are not sorted, they stay at the same position
    /// </summary>
    public partial class MainWindow : Window
    {
        ObservableCollection<Level> levels;

        public MainWindow()
        {
            InitializeComponent();
            lstLevel.Loaded += new RoutedEventHandler(lstLevel_Loaded);
            levels = new ObservableCollection<Level> {
                new Level {
                    Name = "First",
                    Players = new ObservableCollection<User> {
                        new User { Name = "Teddy",  Age = "2" },
                        new User { Name = "Avi",    Age = "19" },
                        new User { Name = "Mayia",  Age = "30" },
                    },
                },
                new Level {
                    Name = "Second",
                    Players = new ObservableCollection<User> {
                                new User { Name = "Brian",  Age = "20" },
                                new User { Name = "Snoopy", Age = "4" }
                            }
                    }
            };
            lstLevel.ItemsSource = levels;
        }

        void lstLevel_Loaded(object sender, RoutedEventArgs e)
        {
            lstLevel.Focus();
            var item = lstLevel.ItemContainerGenerator.ContainerFromIndex(0) as ListBoxItem;
            if (item != null)
                FocusManager.SetFocusedElement(this, item);
        }
    }

    /// <summary>
    /// Level class, items shown in the ListBox.
    /// </summary>
    public class Level : INotifyPropertyChanged
    {
        string name;
        public string Name
        {
            get { return name; }
            set
            {
                name = value;
                OnPropertyChanged(new PropertyChangedEventArgs("Name"));
            }
        }

        ObservableCollection<User> players;
        public ObservableCollection<User> Players
        {
            get { return players; }
            set
            {
                players = value;
                OnPropertyChanged(new PropertyChangedEventArgs("Players"));
            }
        }

        public Level()
        {
        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        public virtual void OnPropertyChanged(PropertyChangedEventArgs e)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, e);
        }

        #endregion
    }

    /// <summary>
    /// User class, items shown in the DataGrid.
    /// </summary>
    public class User : INotifyPropertyChanged
    {
        string name;
        public string Name
        {
            get { return name; }
            set
            {
                name = value;
                OnPropertyChanged(new PropertyChangedEventArgs("Name"));
            }
        }

        String age;
        public string Age
        {
            get { return age; }
            set
            {
                age = value;
                OnPropertyChanged(new PropertyChangedEventArgs("Age"));
            }
        }

        public User()
        {
        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        public virtual void OnPropertyChanged(PropertyChangedEventArgs e)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, e);
        }

        #endregion
    }
}