在LINQ SQL ISNULL等效？ [英] Equivalent of SQL ISNULL in LINQ?

问题描述

在SQL可以运行一个ISNULL（NULL，''）你会如何在LINQ查询做到这一点？

In SQL you can run a ISNULL(null,'') how would you do this in a linq query?

我在此查询联接：

var hht = from x in db.HandheldAssets
        join a in db.HandheldDevInfos on x.AssetID equals a.DevName into DevInfo
        from aa in DevInfo.DefaultIfEmpty()
        select new
        {
        AssetID = x.AssetID,
        Status = xx.Online
        };

但我有一个位类型为非可空（xx.online）我怎么可以设置为false，如果它是空列？

but I have a column that has a bit type that is non nullable (xx.online) how can I set this to false if it is null?

推荐答案

由于 AA 是一组/对象可能为空，你可以检查 AA == NULL ？

Since aa is the set/object that might be null, can you check aa == null ?

（ AA / XX 可能互换（在这个问题一个错字）;对原来的问题举行会谈 XX ，但只定义了 AA ）

(aa / xx might be interchangeable (a typo in the question); the original question talks about xx but only defines aa)

即

select new {
    AssetID = x.AssetID,
    Status = aa == null ? (bool?)null : aa.Online; // a Nullable<bool>
}

或者如果你想默认的是假（不是空）

select new {
    AssetID = x.AssetID,
    Status = aa == null ? false : aa.Online;
}

---

---

更新;在回应downvote，我已经调查了......事实证明，这是正确的做法！下面是关于罗斯文一个例子：

Update; in response to the downvote, I've investigated more... the fact is, this is the right approach! Here's an example on Northwind:

        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);
            }
        }

和这里的TSQL - pretty我们要多东西（它不是 ISNULL ，但它是足够接近）：

And here's the TSQL - pretty much what we want (it isn't ISNULL, but it is close enough):

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？

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