Drag and drop selected item as list box return null

Im using the following code for drag and drop and the following statement

return null when I checked it in the debug,what is wrong here?

var mySelectedItem = listbox1.SelectedItem as ListBoxItem;

  public MainWindow()
        {

            InitializeComponent();
            _UsersList.Add(new User {Name = "Mike"});
            _UsersList.Add(new User { Name = "Nick" });
            listbox1.ItemsSource = _UsersList;

}

        public ObservableCollection<User> userList
        {
            get { return _UsersList; }
        }

   private void listbox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (e.AddedItems.Count > 0 && string.IsNullOrEmpty(textbox1.Text))
            {
                if (listbox1.SelectedItems.Count > 0)
                {
                    var mySelectedItem = listbox1.SelectedItem as ListBoxItem;
                    if (mySelectedItem != null)
                    {
                        DragDrop.DoDragDrop(listbox1, mySelectedItem.Content.ToString(), DragDropEffects.Copy);
                    }
                }
            }
        }

the xaml is

 <ListBox x:Name="listbox1" HorizontalAlignment="Left" Height="115" Margin="100,75,0,0" 
                  VerticalAlignment="Top" Width="150" ItemsSource="{Binding userList}" 
                  SelectionChanged="listbox1_SelectionChanged" >