using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var list = new List<int> {1,2,3,4,5};

        var resultado = Acumular(Combiner, 0, list);

        Console.WriteLine(resultado);

    }

    public static int Acumular(Func<int, int, int> combiner, int nullValue, List<int> list)

    {            

        if (list.Count == 0)

            return 0;

        var primeiro = list.First();

        list.RemoveAt(0);

        return combiner(primeiro, Acumular(combiner, nullValue, list));

    }

    public static int Combiner(int x, int acc)

    {

        return acc + (x*x);    

    }

​

}