属性GET SET使用

        public DateTime? _InvoiceDate
        {
            get { return InvoiceDate == default(DateTime) ? null : new DateTime?(InvoiceDate); }
            set
            {
                if (value.HasValue)
                {
                    if (InvoiceDate != value.Value)
                        InvoiceDate = value.Value;
                }
                else
                    if (InvoiceDate != default(DateTime))
                    InvoiceDate = default(DateTime);

            }
        }