使用 LINQ SUM 时出现异常

Question

我试图获取“bookings”的总和，但收到错误“转换为值类型‘Int32’失败，因为物化值为空。结果类型的通用参数或查询必须使用可为空的类型。”

var bookings = entities.Bookings.Where(x => x.ID == id &&
                                     x.StartDate <= bookingEnd &&
                                     x.EndDate >= bookingStart)
                                    .Sum(x => x.BookingQuantity);

我该如何解决这个问题？如果它的预订为空，我需要得到 0。

Answer 1

尝试使用 null 合并运算符：

var bookings = entities.Bookings.Where(x => x.ID == id &&
                                 x.StartDate <= bookingEnd &&
                                 x.EndDate >= bookingStart && 
                                 x.BookingQuantity != null)
                                .Sum(x => (int?)x.BookingQuantity) ?? 0;

或者将 bookings 声明为可空 int

int？ bookings = ...

编译器类型推断将 Sum 的结果拾取为普通 int，它永远不应该为 null。

Answer 2

此页面建议解决此问题；

Sum(x => (int?)x.BookingQuantity) ?? 0;

Answer 3

添加空值检查。

var bookings = entities.Bookings.Where(x => x.ID == id &&
                                     x.StartDate <= bookingEnd &&
                                     x.EndDate >= bookingStart && 
                                     x.BookingQuantity != null)
                                    .Sum(x => x.BookingQuantity);