<ComboBox ItemsSource="{Binding Items, UpdateSourceTrigger=PropertyChanged, Mode=OneWayToSource}">
            <ComboBoxItem>X</ComboBoxItem>
            <ComboBoxItem>Y</ComboBoxItem>
            <ComboBoxItem>Z</ComboBoxItem>
        </ComboBox> 

        
        private List<ComboBoxItem> _items;
        public List<ComboBoxItem> Items
        {
            get { return _items; }
            set
            {
                _items = value;
                if (value != null)
                    Console.WriteLine(value.Count);
            }
        }
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }