LINQ to Entities查询使用Sum对子对象属性的问题

Question

鉴于此查询:

from s in services
select new
{
    s.Id,
    s.DateTime,
    Class = s.Class.Name,
    s.Location,
    s.Price,
    HeadCount = s.Reservations.Sum(r => r.PartySize), // problem here. r.PartySize is int
    s.MaxSeats
}

如果服务没有任何保留,则抛出此异常:

System.InvalidOperationException:转换为值类型"Int32"失败,因为实现值为null.结果类型的泛型参数或查询必须使用可空类型.

我明白了,但我应该怎么处理呢？我的意图是如果没有预订,则将HeadCount分配为0.

Answer 1

有一个更简单的解决方案:

from s in services
select new
{
    s.Id,
    s.DateTime,
    Class = s.Class.Name,
    s.Location,
    s.Price,
    HeadCount = (int?)s.Reservations.Sum(r => r.PartySize), 
    s.MaxSeats
}

注意演员.这也可能比@ Ahmad的建议产生更简单的SQL.

从本质上讲,你只是在帮助推断类型推理.

Answer 2

你应该检查一下:

HeadCount = s.Reservations != null ? s.Reservations.Sum(r => r.PartySize) : 0,