LINQ Conditional Sum Calculation - cannot be applied to operands of type 'int' and 'bool'

Question

I am trying to perform to calculation. I have a donations (d) table that contains Quantity Needed (d.QtyNeeded) and I need to determine the numbers of items still needed by pulling quantity filled (qtyfilled) from the donors table. Not every donation has a donor, so I need a conditional statement to handle nulls so the sum will work. When I try to compile, I am getting an error: *Operator '-' cannot be applied to operands of type 'int' and 'bool'. I am not great at Linq yet, what am I missing or is there a better way?

QtyOpen = d.QtyNeeded - (from dv in db.Donors
                                    select dv).All(v => v.QtyFilled == null)
                                    ? 0
                                    : (from dv in db.Donations
                                       select dv.QtyFilled).Sum()

Answer 1

The problem is not the LINQ statement, but rather precedence in the subtraction operator. Consider this example:

int result = quantity - true ? 0 : otherValue;

This fails to compile for the exact same reason, Operator '-' cannot be applied to operands of type 'int' and 'bool'. The solution is to simply group the conditional statement in parens:

int result = quantity - (true ? 0 : otherValue);

So, your example should compile by adding parens around your entire conditional operator statement.

Answer 2

Try filtering out the nulls within the initial select by adding a where in the select:

QtyOpen = d.QtyNeeded - (from dv in db.Donors 
                         where dv.QtyFilled != null
                         select dv.QtyFilled).Sum();