Linq 表达式Sum 抛出“不能将 Null 值赋给类型为 System.Decimal (不可为 null 的值类型)的成员”的最佳解决方法

昨天Linq 使用Sum 表达式抛出了“不能将 Null 值赋给类型为 System.Decimal (不可为 null 的值类型)的成员”

var val = dc.DailyData.Where(x => x.ValueTime == DateTime.Now).Sum(x => x.TodayPower);

检查了一下生成的SQL语句，结果如下：

{SELECT SUM([t0].[TodayPower]) AS [value]

FROM [dbo].[DailyData] AS [t0]

WHERE [t0].[ValueTime] = @p0

-- @p0: Input DateTime (Size = 0; Prec = 0; Scale = 0) [2012/3/29 0:00:00]

-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.5420

}

Google了一下，有一个有人提出了强转http://www.cnblogs.com/mine/archive/2009/02/27/1399770.html

于是代码改成如下：

var val = dc.DailyData.Where(x => x.ValueTime == DateTime.Now).Sum(x => (decimal?)x.TodayPower);

不过再检查了生成的SQL语句发现生成的查询语句变成了如下：

{SELECT SUM([t1].[value]) AS [value]

FROM (

SELECT [t0].[TodayPower] AS [value], [t0].[ValueTime]

FROM [dbo].[DailyData] AS [t0]

) AS [t1]

WHERE [t1].[ValueTime] = @p0

-- @p0: Input DateTime (Size = 0; Prec = 0; Scale = 0) [2012/3/29 17:31:18]

-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.5420

}

查了一下MSDN，对于SUM函数重载的说明都基本相同，没有特别之处。最后我尝试了一下使用 as运算符，居然有惊人的发现，生成了最优的SQL语句了。

var val = dc.DailyData.Where(x => x.ValueTime == DateTime.Now).Sum(x =>x.TodayPower as decimal?);

生成的SQL语句如下：

{SELECT SUM([t0].[TodayPower]) AS [value]

FROM [dbo].[DailyData] AS [t0]

WHERE [t0].[ValueTime] = @p0

-- @p0: Input DateTime (Size = 0; Prec = 0; Scale = 0) [2012/3/29 17:47:04]

-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.5420

}