Linq Query Handling Null Values - vb.net

Question

From r In ReceiptLines
Where
r.RECEIPT.RECEIPTDATE >= _reportStartDate
And r.RECEIPT.RECEIPTDATE <= _reportEndDate
Let amount = r.QUANTITY * r.PRICE
Let discount = r.RECEIPTDISCOUNTs.Sum(Function(d) d.DISCOUNT)
where discount > 0
Group By Department = r.ITEMSTYLE.ITEM.CATEGORY.DEPARTMENT.DEPARTMENTNAME
Into Sales = Sum(amount - discount),
Average = Average(amount - discount),
Count = Count()
I am fetching all departments and their sales, average, count from the ReceiptLine, Receipt, ReceiptDiscount tables. The problem i am facing is, if i remove where discount > 0, I am getting null exception. But if I include that, then I only get sales that has discount.
How would I write query that bring all sales less discount (if it has one). Any help is highly appreciated.

Answer 1

This is a common pitfall with LINQ2SQL.
The function SUM in SQL returns null if there are no items in the collection, but the signature of Enumerable.Sum() returns an int. This gives a runtime exception when the SQL query return null where the LINQ2SQL provider expects an integer.
The solution is to cast the result of the sum to a nullable integer and use GetValueOrDefault to convert the null-case to 0.
Replace
Let discount = r.RECEIPTDISCOUNTs.Sum(Function(d) d.DISCOUNT)
with
Let discount = CType(r.RECEIPTDISCOUNTs.Sum(Function(d) d.DISCOUNT), Integer?).GetValueOrDefault(0)

Answer 2

Have you tried:
...
Let amount = r.QUANTITY * r.PRICE
Let nDiscount = r.RECEIPTDISCOUNTs.Sum(Function(d) d.DISCOUNT)
Let discount = IIf(nDiscount == Nothing, 0, nDiscount)
Group By Department = r.ITEMSTYLE.ITEM.CATEGORY.DEPARTMENT.DEPARTMENTNAME
...