Enumerable.Sum() of double to return null if list contains null

Question

var num = new List<double?>{null};  
var result = num.Sum(d => (double?)d);
Console.WriteLine(result is null);

How to make this return true?

You can't make `Sum` do something that it doesn't do. You would need to check first whether there are any non-null items and then only call `Sum` if there are. — jmcilhinney, Jun 07 '23 at 12:56
@TimSchmelter, the OP should be explicit but I interpreted that to be "if list contains ONLY null", i.e. no items with a value. — jmcilhinney, Jun 07 '23 at 12:59
I would fallback to `double.NaN` - https://dotnetfiddle.net/MtDkxK — Rand Random, Jun 07 '23 at 13:27

score 0 · Answer 1 · answered Jun 07 '23 at 12:56

0

I'm don't think it is possible in "one go", but you can do something like this to get the desired result:

double? sum = d.Any((x) => x == null) ? null : d.Sum();

answered Jun 07 '23 at 12:56

Tack D.

2

The question is well-written but I interpreted as meaning if the list ONLY contains `null` then the sum should be `null`. In that case, you'd have to invert your check to look for any items that weren't `null` and switch the outputs. – jmcilhinney Jun 07 '23 at 12:58

1 Answers1