Why is the sum of an empty set null?

Question

If I do:

int updateGamePlays = db.tblArcadeGames.Where(c => c.ParentGameID == GameID).Sum(c => c.Plays);

If no records are returned in this query it throws:

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

The only way to get it to return 0 is by doing:

int updateGamePlays = db.tblArcadeGames.Where(c => c.ParentGameID == GameID).Sum(c => (int?)c.Plays) ?? 0;

In the database c.Plays is a non-nullable int.

In set theory the sum of an empty set should equal 0 (ref). How comes in Linq-to-SQL did they decide to make it return null?

Answer 1

According to a source at Microsoft, Sum() on an empty set is null because of the way it works in SQL:

when the table is empty i´m getting this exception: InvalidOperationException

In SQL, Sum() aggregate operator returns null for an empty set. So this is as designed.

Answer 2

Another alternative is to add a 0 to the set to make sure there's always at least one value.

int updateGamePlays = db.tblArcadeGames.Where(c => c.ParentGameID == GameID)
                                       .Select(c => c.Plays)
                                       .Concat(new [] { 0 })
                                       .Sum();