Building an Agile SQL Data Access Layer

using System;
using System.Collections.Generic;
using System.Text;
using InMemoryDal.Entities;
using InMemoryDal.Interfaces;

namespace InMemoryDal.InMemoryData
{
    [Serializable]
    public sealed class CustomerInMemoryDAL : ICustomerDAL
    {
        internal List<Customer> CustomerList;

        static readonly CustomerInMemoryDAL _istance = new CustomerInMemoryDAL();

        private CustomerInMemoryDAL()
        {
            CustomerList = new List<Customer>();
        }

        internal static CustomerInMemoryDAL Istance
        {
            get { return _istance; }
        }

        #region ICustomerDAL Members

        public List<Customer> Search(Guid id, String firstname, String lastname, String address, CustomerDataAdapterHandler customerDataAdapter)
        {
            List<Customer> lis = CustomerList;

            if (firstname != String.Empty) { lis = lis.FindAll(delegate(Customer entity) { return entity.Firstname == firstname; }); }

            if (id != Guid.Empty) { lis = lis.FindAll(delegate(Customer entity) { return entity.Id == id; }); }

            if (lastname != String.Empty) { lis = lis.FindAll(delegate(Customer entity) { return entity.Lastname == lastname; }); }

            if (address != String.Empty) { lis = lis.FindAll(delegate(Customer entity) { return entity.Address == address; }); }



            return lis;
        }

        #endregion
    }
}