Question

I’m trying to use LINQ to SQL to sum a row in a a resultset returned from a WCF service. I’m able to use LINQ for basic selects, for order by, for filtering, etc. So far so good.

But when I try to Group By / Sum, I get an error message “Value Cannot Be Null” when trying to Take(5) from the query below. (results is non-null and has 1000+s of items, but the query is null when we try to return it:)

EDIT: I’ve removed the invalid cast as mentioned in the answer below, instead selecting the aggregate results into a custom class written to order. Now the page runs without error — but the group by / sum doesn’t work…. I just get the top five highest values in the results, not summed by group. Why?

var query = (from h in results
            group h by h.Order into hh                     
            select new TopOrders()
            {
                Order = hh.Key,
                Total= hh.Sum(r => r.Price)
            }).OrderByDescending(i => i.Total);

return query.Take(5).ToList();

Only possible wrinkle I can point to is that Order is itself a collection (OrderID, OrderName, etc.) and so I’ve tried grouping explicitly on hh.Key.Order.OrderID and so forth, to no effect.

Anything I’m missing from what you can see? Thanks.

Answer 1

It’s probably that r.Price of a few orders is null, so use Sum(r => r.Price ?? 0). But it’s a bit of a guess without knowing your data.

Edit
It’s so obvious that I overlooked it: you try to cast an IEnumerable of an anonymous type to List<TopOrders>, which is not possible. But the as operator produces null if the cast is not valid. So query itself is null! Either remove the cast, or create TopOrders in stead of anonymous types.