c# - Shouldn't sum method be deferred in LINQ

Question

I have the following code:

List<int> no = new List<int>() { 1, 2, 3, 4, 5 };
var res2 = no.Sum(a => a * a);
Console.WriteLine(res2);
no.Add(100);
Console.WriteLine(res2);

I expect the following result:

55
10055

but both are 55

55
55

I have seen here which is about deferred evaluation but was not helpful. Sum is an extension method, but the result is not what I have mentioned. Why?

Answer 1

Only functions that return an IEnumerable<T> can be deferred in Linq (since they can be wrapped in an object that allows deferring).

The result of Sum is an int, so it can't possibly defer it in any meaningful way:

var res2 = no.Sum(a => a * a);
// res2 is now an integer with a value of 55
Console.WriteLine(res2);
no.Add(100);

// how are you expecting an integer to change its value here?
Console.WriteLine(res2);

You can defer the execution (not really defer, but explicitly call it), by assigning the lambda to, for example, a Func<T>:

List<int> no = new List<int>() { 1, 2, 3, 4, 5 };
Func<int> res2 = () => no.Sum(a => a * a);
Console.WriteLine(res2());
no.Add(100);
Console.WriteLine(res2());

This should correctly give 55 and 10055