using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<DoDa> dos = new List<DoDa>();

        dos.Add(new DoDa { Id = 1, amount = 10 });

        dos.Add(new DoDa { Id = 2, amount = 10 });

        dos.Add(new DoDa { Id = 3, amount = 10 });

        dos.Add(new DoDa { Id = 4, amount = 10 });

        dos.Add(new DoDa { Id = 5, amount = 10 });

        var total1 = dos.Sum(t => t.amount);        

        var total2 = dos.Where(t => t.Id < 4).Sum(t => t.amount);

        var total3 = dos.Where(t => t.Id > 10).Sum(t => t.amount);

        Console.WriteLine(total1);

        Console.WriteLine(total2);

        Console.WriteLine(total3);

    }

}

​

public class DoDa

{

    public int Id { get; set;}

    public decimal amount { get; set; }

}