Question

我有以下Linq查询：

(from container in Container
join containerType in ContainerType on container.ContainerType equals containerType
where containerType.ContainerTypeID == someIDValue
select container).Max (row => Convert.ToInt64(row.SerialNumber))

只要至少有一个Container行符合条件，此查询就可以正常运行。如果没有符合条件的行，我会收到以下错误：

The null value cannot be assigned to a member with type System.Int64 which is a non-nullable value type

有没有办法可以重写这个，这样如果没有行满足查询，就会返回一个任意值，比如-1？

Answer 1

您可以将查询的第一部分存储在列表中，然后在列表中执行Max。类似于：

var query = (from container in Container
join containerType in ContainerType on container.ContainerType equals containerType
where containerType.ContainerTypeID == someIDValue
select container);

if(query != null)
int64 maxvalue = query.Max (row => Convert.ToInt64(row.SerialNumber))

（我在飞行中编码，请检查）

Answer 2

如果没有结果，这将给你一个-1：

(
from container in Container
join containerType in ContainerType
    on container.ContainerType equals containerType
where containerType.ContainerTypeID == someIDValue
select container.SerialNumber as long?
).DefaultIfEmpty().Max(sn => sn ?? -1)

Linq查询帮助 - 检测空值

2 个答案: