>Ah. I was thinking that Weight itself was nullable. Try this:
>

var q  = context.EMTransactions.Where(r => r.TransactionHeaders_Id == transactionHeaders_Id).Sum(r => (decimal?) r.Weight);
>// or, if you don't want q to be nullable:
>var q  = context.EMTransactions.Where(r => r.TransactionHeaders_Id == transactionHeaders_Id).Sum(r => (decimal?) r.Weight) ?? 0;
>

>
>
>>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;
>>        }
>>

The sum statement won't evaluate until its called by some means.

Try

decimal detailsWeight = context.TransactionDetails.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id)).Sum(r => r.Weight).ToList();

The .ToList() will force the evaluation of the linq query
.