具有可空和的Linq查询

要使用可为null的Sum形式,请尝试将值强制转换为可为null的值:

from i in Db.Items
select new VotedItem
{
    ItemId = i.ItemId,
    Points = (from v in Db.Votes
              where b.ItemId == v.ItemId
              select v.Points).Sum(r => (decimal?) r.Points)
}

这里将更详细地讨论您的问题:

http://weblogs.asp.net/zeeshanhirani/archive/2008/07/15/applying-aggregates-to-empty-collections-causes-exception-in-linq-to-sql.aspx

from i in Db.Items
select new VotedItem
{
    ItemId = i.ItemId,
    Points = (from v in Db.Votes
              where b.ItemId == v.ItemId
              select v.Points ?? 0).Sum() 
}

from i in Db.Items
select new VotedItem
{
    ItemId = i.ItemId,
    Points = (from v in Db.Votes
              where b.ItemId == v.ItemId)
              .Sum(v => v.Points) 
}

假设“v.Points”为十进制,只需使用以下公式:

from i in Db.Items
select new VotedItem
{
    ItemId = i.ItemId,
    Points = (from v in Db.Votes
              where b.ItemId == v.ItemId
              select (decimal?) v.Points).Sum() ?? 0
}

var count = db.Cart.Where(c => c.UserName == "Name").Sum(c => (int?)c.Count) ?? 0;

因此,问题的根源是SQL查询如下所示:

SELECT SUM([Votes].[Value])
FROM [dbo].[Votes] AS [Votes]
WHERE 1 = [Votes].[UserId] 

如果您不喜欢强制转换为nullabe decimal,您也可以尝试使用Linq to Objects和ToList()方法,

from i in Db.Items
select new VotedItem
{    
  ItemId = i.ItemId,
  Points = (from v in Db.Votes
            where b.ItemId == v.ItemId &&
            v.Points.Count > 0
            select v.Points).Sum()
}

只需在混合中添加另一种方法:)

Where(q=> q.ItemId == b.ItemId && b.Points.HasValue).Sum(q=>q.Points.Value)

我有一个类似的场景,但在求和时我没有比较其他字段。。。

Where(q => q.FinalValue.HasValue).Sum(q=>q.FinalValue.Value);

var sum = Points.DefaultIfEmpty().Sum(c => (Int32)c ?? 0)

我也有同样的问题。使用空列表联合解决了此问题:

List<int> emptyPoints = new List<int>() { 0 };

from i in Db.Items
select new VotedItem
{
 ItemId = i.ItemId,
 Points = (from v in Db.Votes
           where b.ItemId == v.ItemId
           select v.Points).Union(emptyPoints).Sum()
}

如果“Points”为整数,则应能正常工作。

var x = (from a in this.db.Pohybs
                 let sum = (from p in a.Pohybs
                            where p.PohybTyp.Vydej == true
                            select p.PocetJednotek).Sum()
                 where a.IDDil == IDDil && a.PohybTyp.Vydej == false
                 && ( ((int?)sum??0) < a.PocetJednotek)
                 select a);

Where(a => a.ItemId == b.ItemId && !b.IsPointsNull()).Sum(b => b.Points)

        (from i in Db.Items
         where (from v in Db.Votes
                where i.ItemId == v.ItemId
                select v.Points).Count() > 0
         select new VotedItem
         {
             ItemId = i.ItemId,
             Points = (from v in Db.Items
                       where i.ItemId == v.ItemId
                       select v.Points).Sum()
         }).Union(from i in Db.Items
                  where (from v in Db.Votes
                         where i.ItemId == v.ItemId
                         select v.Points).Count() == 0
                  select new VotedItem
                  {
                      ItemId = i.ItemId,
                      Points = 0
                  }).OrderBy(i => i.Points);

这是可行的,但不是很好看或可读。

我有一个类似的问题,我想出了一个解决方案,从数据库中取出我想得到的任何东西,对它们进行计数,然后只有在返回任何东西时才进行求和。由于某些原因,无法让演员组正常工作,因此如果其他人有类似问题,请发布此消息。

例如

Votes = (from v in Db.Votes
          where b.ItemId = v.ItemId
          select v)

If (Votes.Count > 0) Then
    Points = Votes.Sum(Function(v) v.Points)
End If

与前面的答案类似,但也可以将整个总和的结果强制转换为可为null的类型。

from i in Db.Items
select new VotedItem
{
    ItemId = i.ItemId,
    Points = (decimal?)((from v in Db.Votes
              where b.ItemId == v.ItemId
              select v.Points).Sum()) ?? 0
}

可以说,这更符合实际情况,但它的效果与演员阵容相同 this answer .