Web Service via Data Access Object Pattern

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Collections.Generic;
using System.IO;
using System.Xml.Serialization;
using System.Xml;
using AndrewGolik.DaoTraining.TransferObjects;
using AndrewGolik.DaoTraining.Common.Utilities;

namespace AndrewGolik.DaoTraining.DataAccessLayer.XmlRepository 
{
    public class XmlOrderDao:OrderDao
    {
        private Object thisLock = new Object();

        private bool IsCustomerExist(int customerID) 
        {
            XmlDocument xmlCustomers = new XmlDocument();
            xmlCustomers.Load(XmlDaoFactory.PathToCustomersFile);
            if (xmlCustomers.DocumentElement.SelectSingleNode("Customer[CustomerID="+customerID+"]") != null)
                return true;
            else
                return false;
            
        }

        public int CreateOrder(Order order) 
        {

            lock (thisLock)
            {
                if (!IsCustomerExist(order.CustomerID))
                    throw new ArgumentException("The customerID is invalid.");

                XmlDocument xmlOrders = new XmlDocument();
                xmlOrders.Load(XmlDaoFactory.PathToOrdersFile);
                //generate the new id for the order
                XmlNode xmlLastGeneratedId = xmlOrders.DocumentElement.Attributes.GetNamedItem("LastGeneratedId");
                int lastGeneratedIdValue = Convert.ToInt32(xmlLastGeneratedId.Value) + 1;
                xmlLastGeneratedId.InnerText = lastGeneratedIdValue.ToString();
                //assign the new generated id to the order then serialize it to xml
                order.OrderID = lastGeneratedIdValue;
                XmlDocument xmlOrder = Serialization.FillXmlDocFromObject<Order>(order);
                //import new order xml to orders list
                xmlOrders.DocumentElement.AppendChild(xmlOrders.ImportNode(xmlOrder.DocumentElement, true));
                xmlOrders.Save(XmlDaoFactory.PathToOrdersFile);
                return lastGeneratedIdValue;

            }

           
       }
       
       public bool DeleteOrder(int orderID) 
       {
           lock (thisLock)
           {
               XmlDocument xmlDoc = new XmlDocument();
               xmlDoc.Load(XmlDaoFactory.PathToOrdersFile);
               XmlNode xmlOrderNode = xmlDoc.DocumentElement.SelectSingleNode("Order[OrderID = " + orderID + "]");
               if (xmlOrderNode != null)
               {
                   xmlOrderNode.ParentNode.RemoveChild(xmlOrderNode);
                   xmlDoc.Save(XmlDaoFactory.PathToOrdersFile);
                   return true;
               }
               else
                   return false;
           }

       }
        public bool UpdateOrder(Order order)
        {

            lock (thisLock)
            {   //first check if customer id passed exists
                if(!IsCustomerExist(order.CustomerID))
                    throw new ArgumentException("The customerID is invalid.");

                if (DeleteOrder(order.OrderID))
                {
                    XmlDocument xmlOrder = Serialization.FillXmlDocFromObject<Order>(order);
                    //import order xml updated to orders list
                    XmlDocument xmlOrders = new XmlDocument();
                    xmlOrders.Load(XmlDaoFactory.PathToOrdersFile);
                    xmlOrders.DocumentElement.AppendChild(xmlOrders.ImportNode(xmlOrder.DocumentElement, true));
                    xmlOrders.Save(XmlDaoFactory.PathToOrdersFile);
                    return true;
                }
                else

                    return false;
            }
        }

        public List<Order> GetOrders()
        {

            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load(XmlDaoFactory.PathToOrdersFile);
            XmlNodeList xmlOrderList = xmlDoc.DocumentElement.SelectNodes("Order");
            List<Order> orders = new List<Order>();
            for (int index = 0; index < xmlOrderList.Count; index++)
            {
                XmlDocument xmlOrder = new XmlDocument();
                xmlOrder.LoadXml(xmlOrderList[index].OuterXml);
                Order order = Serialization.FillObjectFromXMLDoc<Order>(xmlOrder);
                orders.Add(order);

            }
            return orders;
        }
       


    }
}