public abstract class TreeviewMenuItem
{
    public virtual string Text { get; set; }
    public virtual string DisplayName { get => Text; }
    public virtual MenuSubmenu ParentMenu { get; set; } = null;
}

public class MenuSubmenu : TreeviewMenuItem
{
    public override string DisplayName { get => Text + " [" + Items.Count + "]"; }
    public List<MenuItem> Items { get; set; }

    public MenuSubmenu(MenuSubmenu parent = null)
    {
        ParentMenu = parent;
        Items = new List<MenuItem>();
    }
}

public class MenuItem : TreeviewMenuItem
{
    public MenuItem(MenuSubmenu parent = null)
    {
        ParentMenu = parent;
    }
}