如何将observablecollection放到List中

我想把当前observablecollection数据在列表上的按钮单击事件。下面是c#代码，但它给出了错误:对象引用未设置为对象的实例。

    ObservableCollection<CheckInData> _CheckInCollection = new ObservableCollection<CheckInData>();

    public ObservableCollection<CheckInData> CheckInCollection
    {
        get { return _CheckInCollection; }
    }
    public class CheckInData
    {
        public string RoomNumber { get; set; }
        public decimal Price { get; set; }
        public string Currecny { get; set; }
        public decimal Discount { get; set; }
        public string CheckOut { get; set; }
        public int TotalDay { get; set; }
        public decimal TotalPrice { get; set; }
        public int CheckOutYear { get; set; }
        public int CheckOutMonth { get; set; }
        public int CheckOutDay { get; set; }
        public Boolean IncToday { get; set; }
    }
    private void btnPrintInvoice_Click(object sender, RoutedEventArgs e)
    {
        DataToExcel.Invoice inv = new DataToExcel.Invoice();
        inv._BilledTo = Guest[0];
        foreach (CheckInData coll in _CheckInCollection)
        {
            for (int i = 0; i < _CheckInCollection.Count; i++)
            {
                inv._RoomPrice.Add(coll.RoomNumber[i].ToString());
            }
        }
    }