CodeBehind.cs

private ObservableCollection<VMCopyToMenu> Menus;

        /// <summary>
        /// 
        /// </summary>
        public TreeViewWindow()
        {
            InitializeComponent();
            Menus = new ObservableCollection<VMCopyToMenu>();
            LoadMenuItems();
        }

        /// <summary>
        /// 
        /// </summary>
        private void LoadMenuItems()
        {
            var menus = new List<VMCopyToMenu>()
            {
                new VMCopyToMenu()
                {
                    MainMenuItemName = "Item 1",
                    MenuItems = GetSampleData()
                },

                new VMCopyToMenu()
                {
                    MainMenuItemName = "Item 2",
                    MenuItems = GetSampleData()
                },
            };

            Menus = new ObservableCollection<VMCopyToMenu>(menus);
            //SampleTree.ItemsSource = Menus;
            SampleTree2.ItemsSource = Menus;
        }

        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        private ObservableCollection<VMCopyToMenuItem> GetSampleData()
        {
            var list = new List<VMCopyToMenuItem>()
            {
                new VMCopyToMenuItem()
                {
                    MenuItemName = "Sub Item 1",
                    SubMenuItems = GetAnotherSampleData()
                },
                new VMCopyToMenuItem()
                {
                    MenuItemName = "Sub Item 2",
                    SubMenuItems = GetAnotherSampleData()
                }
            };

            return new ObservableCollection<VMCopyToMenuItem>(list);
        }

        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        private ObservableCollection<VMCopyToMenuItem> GetAnotherSampleData()
        {
            var list = new List<VMCopyToMenuItem>()
            {
                new VMCopyToMenuItem()
                {
                    MenuItemName = "Sub-Sub Item 1"
                },
                new VMCopyToMenuItem()
                {
                    MenuItemName = "Sub-Sub Item 2"
                }
            };

            return new ObservableCollection<VMCopyToMenuItem>(list);
        }