Linq query with nullable sum

问题

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()
}

I got this query, however it fails if no votes are found with exception:

The null value cannot be assigned to a member with type System.Int32 which is a non-nullable value type.

I assume its because sum returns an int and not a nullable int, giving sum a int? as input only give the same error, probably cause sum only workes on ints.

Any good workaround for this?

回答1:

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() 
}

EDIT - ok what about this... (Shooting again since I don't know your model...):

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) 
}

回答2:

You want to use the nullable form of Sum, so try casting your value to a nullable:

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)
}

Your issue is discussed here in more detail:

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

回答3:

Assuming "v.Points" is a decimal just use the following:

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
}

回答4:

If you don't like casting to nullabe decimal you could also try using Linq To Objects with ToList() method,

LinqToObjects Sum of empty collection is 0, where LinqToSql Sum of empty collection is null.

回答5:

Try to check this out:

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

So, the root of the problem is that SQL query like this:

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

returns NULL

回答6:

A simple but effective workaround would be to only sum the votes where Points.Count > 0, so you never have null values:

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()
}

回答7:

Just to add another method into the mix :)

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

I had a similar scenario but I wasn't comparing an additional field when summing...

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

回答8:

Assuming Points is a List of Int32, how about something like:

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

回答9:

I had the same problem. Solved it with empty list union:

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()
}

In case of "Points" is integer this should work.

回答10:

I think this is the same case. I resolved it. This is my solution:

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);

I hope this help.

回答11:

        (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);

This works, but isn't very pretty or readable.

回答12:

I had a similar issue and came up with the solution of getting whatever I was trying to get out of the database, do a count on those and then only if I had anything returned do a sum. Wasn't able to get the cast working for some reason so posting this if anyone else had similar issues.

e.g.

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

And then check to see if you've got any results so that you don't get null returned.

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

回答13:

Similar to previous answers, but you can also cast the result of the entire sum to the nullable type.

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
}

Arguably this better fits what is really going on but it has the same effect as the cast in this answer.

回答14:

Thought I would throw another solution out there. I had a similar issue and this is how I ended up solving it:

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

来源：https://stackoverflow.com/questions/696431/linq-query-with-nullable-sum

标签

linq

linq-to-sql

sum

nullable