由于aa
是集/可能为空的对象,你可以检查aa == null
?
(aa
/xx
可能互换(在这个问题一个错字);原来的问题谈到xx
但只定义aa
)
即
select new {
AssetID = x.AssetID,
Status = aa == null ? (bool?)null : aa.Online; // a Nullable<bool>
}
,或者如果你希望默认是false
(不null
):
select new {
AssetID = x.AssetID,
Status = aa == null ? false : aa.Online;
}
更新;为了回应downvote,我调查了更多......事实是,这是正确的做法!下面是关于罗斯文一个例子:
using(var ctx = new DataClasses1DataContext())
{
ctx.Log = Console.Out;
var qry = from boss in ctx.Employees
join grunt in ctx.Employees
on boss.EmployeeID equals grunt.ReportsTo into tree
from tmp in tree.DefaultIfEmpty()
select new
{
ID = boss.EmployeeID,
Name = tmp == null ? "" : tmp.FirstName
};
foreach(var row in qry)
{
Console.WriteLine("{0}: {1}", row.ID, row.Name);
}
}
而这里的TSQL - 几乎是我们想要的东西(它不是ISNULL
,但它是足够接近):
SELECT [t0].[EmployeeID] AS [ID],
(CASE
WHEN [t2].[test] IS NULL THEN CONVERT(NVarChar(10),@p0)
ELSE [t2].[FirstName]
END) AS [Name]
FROM [dbo].[Employees] AS [t0]
LEFT OUTER JOIN (
SELECT 1 AS [test], [t1].[FirstName], [t1].[ReportsTo]
FROM [dbo].[Employees] AS [t1]
) AS [t2] ON ([t0].[EmployeeID]) = [t2].[ReportsTo]
-- @p0: Input NVarChar (Size = 0; Prec = 0; Scale = 0) []
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.1
QED?
你的意思是“aa.Online”? “xx”没有在任何地方定义... – 2009-01-05 14:35:24