using System;

using System.Collections.Generic;

using System.Linq;

using System.Diagnostics;

public class Program

{

    public static void Main()

    {

        Stopwatch sw = new Stopwatch();

        sw.Start();

        var shipments = new List<Shipment>();

        shipments.Add(new Shipment() { Number = 0 });

        shipments.Add(new Shipment() { Number = 1 });

        shipments.Add(new Shipment() { Number = 2 });

        shipments.Add(new Shipment() { Number = 3 });

        shipments.Add(new Shipment() { Number = 4 });

        shipments.Add(new Shipment() { Number = 5 });

        DoSomething(shipments);

        sw.Stop();

        Console.WriteLine(sw.ElapsedTicks);

        sw.Reset();

        sw.Start();

        var shipments2 = new List<Shipment>();

        shipments2.Add(new Shipment() { Number = 0 });

        shipments2.Add(new Shipment() { Number = 1 });

        shipments2.Add(new Shipment() { Number = 2 });

        shipments2.Add(new Shipment() { Number = 3 });

        shipments2.Add(new Shipment() { Number = 4 });

        shipments2.Add(new Shipment() { Number = 5 });

        DoSomethingWitAList(shipments2);

        sw.Stop();

        Console.WriteLine(sw.ElapsedTicks);

    }

    public static void DoSomething(IEnumerable<Shipment> items)

    {

        if(items.Count() == 1)

        {

            Console.WriteLine("item number " + items.First().Number);

            return;

        }

        var baseItem = items.First();

        Console.WriteLine("base item number " + baseItem.Number);

        for(int i = 1; i < items.Count(); i++)

            Merge(baseItem, items.ElementAtOrDefault(i));           

        Console.WriteLine("base item number " + baseItem.Number);

    }

    public static void DoSomethingWitAList(IEnumerable<Shipment> items)

    {

        var list = items.ToList();

        if(list.Count == 1)

        {

            Console.WriteLine("item number " + items.First().Number);

            return;

        }

        var baseItem = list[0];

        Console.WriteLine("base item number " + baseItem.Number);

        for(int i = 1; i < list.Count; i++)

            Merge(baseItem, list[i]);

        Console.WriteLine("base item number " + baseItem.Number);

    }

    private static void Merge(Shipment baseItem, Shipment toMerge)

    {

        baseItem.Number += toMerge.Number;

    }

}

​

public class Shipment

{

    public int Number {get;set;}

}