实体框架-使用LINQ SUM时异常

Question

我试图获得"预订"的总和，我得到错误"转换到值类型'Int32'失败，因为物化值为空。结果类型的泛型参数或查询必须使用可空类型。"

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?如果它为空，我需要得到0否则它的预订

Answer 1

尝试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;

或将预订声明为可空int

int ?

编译器的类型推断将Sum的结果提取为普通int，该int不应该为null。

Answer 2

此页面建议修复此问题;

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

Answer 3

添加null检查。

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