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Linq sum force to return null

I suppose the answer is trivial:

List<int?> intList = new List<int?>(){null, null, null};
int? sum = list.Sum();
int sum2 = list.Sum(a => a.HasValue ? a.Value : 0);
int sum3 = list.Sum().Value;

The sum always returns 0, why is zero necessary at all? Is there a way to make linq sum return null? What am I missing here?

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5 answers


Here is the implementation of Sum ()

public static int? Sum(this IEnumerable<int?> source) {
if (source == null) throw Error.ArgumentNull("source");
int sum = 0;
checked {
    foreach (int? v in source) {
        if (v != null) sum += v.GetValueOrDefault();
    }
}
return sum;


The reason for not returning null is the way it is implemented - the int sum = 0;

resulting usage can never return null.

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It just matches your input type. If you change your list to List it will return int not int? Since your list is int? this is what it returns ...



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You can try this:

int? sum = intList.TrueForAll(x => x == null) ? null : intList.Sum();
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As fubo already wrote:

The reason for rejecting a null value is the way it is implemented - using it int sum = 0;

as a result can never return null.

Why not write your own extension method 

public static int? NullableSum( this IEnumerable<int?> source)
{
    int? sum = null;
    foreach (int? v in source)
    {
        if (v != null)
        {
            if (sum == null)
            {
                sum = 0;
            }

            sum += v.GetValueOrDefault()               
        }
    }

    return sum;
}
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You can also try this if your linq sum returns null:

int? sum = intList.AsQueryable().Any(a => a.HasValue) ? intList.AsQueryable().Sum(a => a.Value) : (int?)null;
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