How to force LINQ Sum() to return 0 while source collection is empty

Question

Try changing your query to this:

db.Leads.Where(l => l.Date.Day == date.Day
            && l.Date.Month == date.Month
            && l.Date.Year == date.Year
            && l.Property.Type == ProtectedPropertyType.Password
            && l.Property.PropertyId == PropertyId)
         .Select(l => l.Amount)
         .DefaultIfEmpty(0)
         .Sum();

This way, your query will only select the Amount field. If the collection is empty, it will return one element with the value of 0 and then the sum will be applied.

I prefer to use another hack:

double earnings = db.Leads.Where(l => l.Date.Day == date.Day
                                      && l.Date.Month == date.Month
                                      && l.Date.Year == date.Year
                                      && l.Property.Type == ProtectedPropertyType.Password
                                      && l.Property.PropertyId == PropertyId)
                          .Sum(l => (double?) l.Amount) ?? 0;

Try this instead, it's shorter:

db.Leads.Where(..).Aggregate(0, (i, lead) => i + lead.Amount);

That's win for me:

int Total = 0;
Total = (int)Db.Logins.Where(L => L.id == item.MyId).Sum(L => (int?)L.NumberOfLogins ?? 0);

In my LOGIN table, in field NUMBEROFLOGINS some values are NULL and others have an INT number. I sum here total NUMBEROFLOGINS of all users of one Corporation (Each Id).

Try:

double earnings = db.Leads.Where(l => l.ShouldBeIncluded).Sum(l => (double?) l.Amount) ?? 0;

The query "SELECT SUM([Amount])" will return NULL for empty list. But if you use LINQ it expects that the "Sum(l => l.Amount)" returns double and it doesn't allow you to use "??" operator to set 0 for empty collection.

In order to avoid this situation you need to make LINQ expect "double?". You can do it by casting "(double?)l.Amount".

It doesn't affect the query to SQL but it makes LINQ working for empty collections.