Hi Viv.

>If you want null results to be treated as 0 then you could just use : Sum(r=>r.Weight ?? 0)

Doesn't compile either.

This on the other hand does work. Interestingly q does not return null if there no transactions in EM. So why won't it sum()?

        public static decimal ComputeWeightDifference(int transactionHeaders_Id,PICSEntities context)
        {
            decimal emWeight = 0;
            var q = context.EMTransactions.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id));
            if ((q.Count() == 0) == false) { emWeight = q.Sum(r => r.Weight); }
            decimal detailsWeight = context.TransactionDetails.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id)).Sum(r => r.Weight);
            return emWeight   - detailsWeight;
        }