Equivalent of SQL ISNULL in LINQ?

Question

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

I have a join in this query:

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
        };

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?

Answer 1

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

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

ie

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

or if you want the default to be false (not null ):

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

---

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);
            }
        }

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?

Answer 2

You can use the ?? operator to set the default value but first you must set the Nullable property to true in your dbml file in the required field ( xx.Online )

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 ?? false
        };

Answer 3

I often have this problem with sequences (as opposed to discrete values). If I have a sequence of ints, and I want to SUM them, when the list is empty I'll receive the error "InvalidOperationException: The null value cannot be assigned to a member with type System.Int32 which is a non-nullable value type.".

I find I can solve this by casting the sequence to a nullable type. SUM and the other aggregate operators don't throw this error if a sequence of nullable types is empty.

So for example something like this

MySum = MyTable.Where(x => x.SomeCondtion).Sum(x => x.AnIntegerValue);

becomes

MySum = MyTable.Where(x => x.SomeCondtion).Sum(x => (int?) x.AnIntegerValue);

The second one will return 0 when no rows match the where clause. (the first one throws an exception when no rows match).

Answer 4

看起来类型是布尔值，因此永远不能为null，默认情况下应该为false。