Linq .Sum() function fails when there is nothing to Sum

Question

When running the following Linq query

ViewBag.AmountThisYear = db.Bookings
            .Where(p => p.Id == id && 
                        p.StartDate.Year == DateTime.Now.Year)
            .Sum(t => t.Price);

I am getting the following error when there are no results returned in the where clause

The cast to value type 'System.Decimal' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

How should the Sum be written to cope with this situation

Answer 1

Since no rows are returned you cannot sum. You could use DefaultIfEmpty:

ViewBag.AmountThisYear = db.Bookings
            .Where(p => p.Id == id && 
                        p.StartDate.Year == DateTime.Now.Year)
            .Select(t => t.Price)
            .DefaultIfEmpty(0)
            .Sum();

Answer 2

If the calculating field is not nullable, then you need to cast nullable first for calculation. So the change will be like this below:

ViewBag.AmountThisYear = db.Bookings
            .Where(p => p.Id == id && 
                        p.StartDate.Year == DateTime.Now.Year)
            .Sum(t => (decimal?)t.Price) ?? 0m;

Also, add coalesce operator (??) to translate null to 0.