c# array of objects how to sum object.a property based on object.b property

I’m using c# .NET 4.8. I have an array of objects. The objects have properties, I want to get the sum of all of one of the properties only if the other property is not null.

So if I have these 3 objects in an array:

myObject[] objectArray = [
    new myObject{ a = 1, b = 2 },
    new myObject{ a = 3 },
    new myObject{ a = 4, b = 5 }
];

I want to sum myObject.a only if myObject.b is not null, which in this case would be 5 (1 from objectArray[0].a and 4 from objectArray[2].a).

I’m trying to accomplish this with linq but am having an issue with figuring out the syntax to it.

I’ve tried this:

objectArray.Select(x => x.a).Where(x.b != null).Sum();

but trying to reference x.b in my Where statement gives me the following error:

Cannot resolve symbol ‘x’

I’m stuck on how to format this linq query to get the value I need. If anyone could give me some instruction on how to accomplish what I’ve outlined in this question I would greatly appreciate it. A correct, clearly explained answer will be marked as accepted and upvoted. Thanks.

>Solution :

You have forgotten that a Where expression receives the current item in the sequence enumerated. So it should be Where(x => x.b != null) but you need also to tell the Sum expression what you want to sum. The Select part is not needed at all.

So, assuming b is a nullable type (IE int? b {get;set;}) then you can get the sum of a where b is null in a simple way

var result = objectArray.Where(x => x.b != null).Sum(k => k.a);

If b is not a nullable type (IE int b {get;set;} ) then, when you don’t give it a value, it has the default value for integers (zero) so the sum could be done without any where

var result = objectArray.Sum(k => k.a);