c# - How to use self defined aggregate function with LINQ? (without using extension methods)

Question

In the question Calculate the sum of numbers from 1 to n using LINQ, the function sum was preexisting, what about when we want to define our own function ? for example SquareSum function, where the the square of numbers are to be added, or some other non preexisting function?

Edit : the use of extension method was commented, I was actually looking for a way to do it without extension methods.

Answer 1

You can write it like this

IEnumerable<int> range = Enumerable.Range(1, n);
var sum = range.Aggregate(0, (x, y) => x + y);

Sum of squares:

IEnumerable<int> range = Enumerable.Range(1, n);
var sum = range.Aggregate(0, (x, y) => x + y * y);