/// <summary>
        /// Adds a new element to the combo
        /// </summary>
        /// <param name="item">item to add</param>
        /// <param name="index">position to adding</param>
        public int AddItem(object item, int index = -1)
        {
            var newItem = CreateItem(item);

            if (index == -1 || index == ListBoxItems.Count)
            {
                ListBoxItems.Add(newItem);
                return(ListBoxItems.Count - 1);
            }
            ListBoxItems.Insert(index, newItem);
            return(index);
        }
 private void OnAddItemCommandExecute()
 {
     ListBoxItems.Insert(0, CreateUser());
 }