c# - LINQ 中 SQL ISNULL 的等价物？

Question

在 SQL 中，您可以运行 ISNULL(null,'') 在 linq 查询中如何执行此操作？

我在这个查询中有一个连接:

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
        };

但我有一个列的位类型不可为空 (xx.online) 如果它为空，我如何将其设置为 false？

Answer 1

由于 aa 是可能为 null 的集合/对象，您可以检查 aa == null 吗？

(aa/xx 可能可以互换(问题中的错字)；原始问题讨论的是 xx 但仅定义了 aa)

即

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

或者如果您希望默认值为 false(而不是 null):

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

---

更新；作为对否决票的回应，我调查了更多……事实是，这是正确的方法!下面是 Northwind 上的一个例子:

        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);
            }
        }

这里是 TSQL - 几乎是我们想要的(它不是 ISNULL，但足够接近):

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？