LINQ Aggregate behavior of nullable types

Question

Can someone explain what is going on here? Howcome both these things are true?

[TestMethod]
public void WhatIsGoingOnHere()
{
    List<int?> list = new List<int?> { 1, 2, 3, null, 5, 6 };
    Assert.AreEqual(17, list.Sum());

    int? singleSum = 1 + 2 + 3 + null + 5 + 6;

    Assert.IsNull(singleSum);
}

Specifically, why does the Sum() method not return 'null'? Or the singleSum not equal 17?

Answer 1

The .Sum() method of nullable types ignores all null values:

MSDN: Enumerable.Sum Method (IEnumerable<Nullable<Int32>>)

Remarks

The result does not include values that are null.

Whereas if you add any number via + to null the result is null.

As Andrew Hare noted, it doesn't make sense to add a number to null: null is not 0; it is just not a number.

Answer 2

What you are seeing is the difference between using Enumerable.Sum and actually adding the values yourself.

The important thing here is the null is not zero. At first glance you would think that singleSum should equal 17 but that would mean that we would have to assign different semantics to null based on the data type of the reference. The fact that this is an int? makes no difference - null is null and should never be semantically equal with the numeric constant 0.

The implementation of Enumerable.Sum is designed to skip over any value that is null in the sequence so that is why you are seeing the different behavior between the two tests. However the second test rightly returns null as the compiler is smart enough to know that adding anything to null yields null.

Here is the implementation of Enumerable.Sum that accepts a parameter of int?:

public static int? Sum(this IEnumerable<int?> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    int num = 0;
    foreach (int? nullable in source)
    {
        // As you can see here it is explicitly designed to
        // skip over any null values
        if (nullable.HasValue)
        {
            num += nullable.GetValueOrDefault();
        }
    }
    return new int?(num);
}