A linq-to-sql query isn't executed as code, but rather translated into SQL. Sometimes this is a "leaky abstraction" that yields unexpected behaviour.
One such case is null handling, where there can be unexpected nulls in different places.
...DefaultIfEmpty(0).Sum(0) can help in this (quite simple) case, where there might be no elements and sql's
null whereas c# expect 0.
A more general approach is to use
?? which will be translated to
COALESCE whenever there is a risk that the generated SQL returns an unexpected null:
var creditsSum = (from u in context.User
join ch in context.CreditHistory on u.ID equals ch.UserID
where u.ID == userID
select (int?)ch.Amount).Sum() ?? 0;
This first casts to
int? to tell the C# compiler that this expression can indeed return
null, even though
Sum() returns an
int. Then we use the normal
?? operator to handle the
Based on this answer, I wrote a blog post with details for both LINQ to SQL and LINQ to Entities.