class Bill
{
    private string billNumber;
    private string billDate;
    private DateTime date;
    private string from;
    private string to;
    private string billCost;
    private string power1;
    private string power2;
    private string power3;
    private string power4;
    private string power5;
    private string power6;
    private string contractNumber;
}

class Contract
{
    private List<Bill> billList;
    private Dictionary<double, Bill> billsDictionary;
    private double contractNumber;
}

class Pet
        {
            public string Name { get; set; }
            public int Age { get; set; }
        }

        public static void OrderByEx1()
        {
            Pet[] pets = { new Pet { Name="Barley", Age=8 },
                           new Pet { Name="Boots", Age=4 },
                           new Pet { Name="Whiskers", Age=1 } };

            IEnumerable<Pet> query = pets.OrderBy(pet => pet.Age);

            foreach (Pet pet in query)
            {
                Console.WriteLine("{0} - {1}", pet.Name, pet.Age);
            }
        }

        /*
         This code produces the following output:

         Whiskers - 1
         Boots - 4
         Barley - 8
        */

public class Comp : IComparer<Bill>
{
    public int Compare(Bill x, Bill y)
    {
       // remember to handle null values first (x or y or both are nulls)
       return x.date.CompareTo(y.date);
    }
}

billList.Sort(new Comp());

billList = billList.OrderBy(bill => bill.date).ToList();