How to do Linq aggregates when there might be an empty set?

Question

I have a Linq collection of Things, where Thing has an Amount (decimal) property.

I'm trying to do an aggregate on this for a certain subset of Things:

var total = myThings.Sum(t => t.Amount);

and that works nicely. But then I added a condition that left me with no Things in the result:

var total = myThings.Where(t => t.OtherProperty == 123).Sum(t => t.Amount);

And instead of getting total = 0 or null, I get an error:

System.InvalidOperationException: The null value cannot be assigned to a member 
with type System.Decimal which is a non-nullable value type.

That is really nasty, because I didn't expect that behavior. I would have expected total to be zero, maybe null - but certainly not to throw an exception!

What am I doing wrong? What's the workaround/fix?

EDIT - example

Thanks to all for your comments. Here's some code, copied and pasted (not simplified). It's LinqToSql (perhaps that's why you couldn't reproduce my problem):

var claims = Claim.Where(cl => cl.ID < 0);
var count = claims.Count(); // count=0
var sum = claims.Sum(cl => cl.ClaimedAmount); // throws exception

Answer 1

It almost seems better to stick with something as simple as

decimal total = decimal.Zero;

foreach (Thing myThing in myThings) {
    if (myThing.OtherProperty == 123) {
        total = total + myThing.Amount;
    }
}

Except, this example works for me (as suggested by Craig)

Using this class...

public class Location
{
    public string Map { get; set; }
    public int Top { get; set; }
    public int Left { get; set; }
}

And this set up...

        List<Location> myThings = new List<Location>();
        myThings.Add(new Location()
        {
            Map = "A",
            Top = 10,
            Left = 10
        });

        var total = myThings.Where(t => t.Map == "B").Sum(t => t.Top);

Get's you a total of 0.

Answer 2

If t has a property like a 'HasValue', then I would change the expression to:

var total = 
     myThings.Where(t => (t.HasValue) && (t.OtherProperty == 123)).Sum(t => t.Amount); 

Answer 3

it throws an exception because the result of the combined sql query is null and this cant be assigned to the decimal var. If you did the following then your variable would be null (I assume ClaimedAmount is decimal):

var claims = Claim.Where(cl => cl.ID < 0);
var count = claims.Count(); // count=0
var sum = claims.Sum(cl => cl.ClaimedAmount as decimal?);

then you should get the functionality you desire.

You could also do ToList() at the point of the where statement and then the sum would return 0 but that would fall foul of what has been said elsewhere about LINQ aggregates.

Answer 4

I can reproduce your problem with the following LINQPad query against Northwind:

Employees.Where(e => e.EmployeeID == -999).Sum(e => e.EmployeeID)

There are two issues here:

1. Sum() is overloaded
2. LINQ to SQL follows SQL semantics, not C# semantics.

In SQL, SUM(no rows) returns null, not zero. However, the type inference for your query gives you decimal as the type parameter, instead of decimal?. The fix is to help type inference select the correct type, i.e.:

Employees.Where(e => e.EmployeeID == -999).Sum(e => (int?)e.EmployeeID)

Now the correct Sum() overload will be used.