using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var w = GetAS();

        Print(w);

    }

    private static IEnumerable<int> GetAS() => X.GetAccumulatingSum(null);

    private static void Print(IEnumerable<int> s) 

    {

        foreach(var v in s)

            Console.WriteLine(v);

    }           

}

​

​

public static class X

{

    public static IEnumerable<int> GetAccumulatingSum(this IEnumerable<int> source)

    {

        if(source == null)

            throw new ArgumentNullException(nameof(source));

        var s = 0;

        foreach(var v in source)

            yield return s += v;

​

    }

}

​