在SQL中,您可以运行ISNULL(null,’‘),如何在linq查询中执行此操作?
我在此查询中加入:
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 };
但是我有一个列的位类型不可为空(xx.online),如果它为null,如何将其设置为false?
由于aa集合/对象可能为空,您可以检查aa == null吗?
aa
aa == null
(aa/ xx可能是可互换的(问题中的错字);最初的问题xx只是在谈论但仅定义aa)
xx
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(not null):
false
null
select new { AssetID = x.AssetID, Status = aa == null ? false : aa.Online; }
更新;为了应对下降的投票,我进行了更多调查……事实是,这是正确的方法!这是罗斯文的一个例子:
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); } }
这是TSQL-我们几乎想要的(不是ISNULL,但是足够接近):
ISNULL
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?