Layers Pattern in Practice

using System;
using System.Collections.Generic;
using System.Linq;
using BillsDalLib;
using BillsEntityLib;
using System.Diagnostics;

namespace BillsBusinessLogicLib
{
    /// <summary>
    /// BillManager class for business logic management
    /// </summary>
    public class BillsManager : IBillsManager
    {
        private static BillsManager _manager;
        private readonly IDalBillsManager _dalManager;
        private static object lockObject = new object();
        private static BooleanSwitch _bSwitch;
        #region Constructor
        private BillsManager()
        {
            _dalManager = XmlDalBillsManager.GetInstance(); //set underlying DAL object to XML Based data storage
            _bSwitch = new BooleanSwitch("BooleanSwitch", "Switch from config file"); //if no such switch in config file _bSwitch == false
            if(_bSwitch.Enabled)
                Trace.TraceInformation("Singleton Instance Created:" + this.GetType().ToString());
        }
        /// <summary>
        /// Private constructor [Singleton class]
        /// </summary>
        private BillsManager(params object [] settings) : this()
        {        
            _dalManager.Settings = settings;
        }
        #endregion
        #region Singleton
        /// <summary>
        /// Gets the singleton instance of BillsManager class
        /// </summary>
        /// <returns>BillsManager object</returns>
        public static BillsManager GetInstance()
        {
            if (_manager == null)   //lazy initialization
            {
                lock (lockObject)
                {
                    if (_manager == null)
                    {
                        _manager = new BillsManager();
                    }
                }
            }
            return _manager;
        }
        #endregion
        #region IBillsManager Members
        /// <summary>
        /// Reads recent data from the datasource
        /// </summary>
        /// <returns>List of recent bills</returns>
        public Bills Read(DateTime fromDate, DateTime toDate)
        {
            try
            {
                return _dalManager.Read(fromDate, toDate);
            }
            catch (DalBillManagerException e)
            {
                throw new BillManagerException(e.Message, e);
            }
        }

        /// <summary>
        /// Read an item from the datasource by its ID
        /// </summary>
        /// <param name="guid">Bill to read</param>
        /// <returns>Bill object</returns>
        public Bill ReadById(Guid guid)
        {
            if (Guid.Empty == guid)
                throw new ArgumentException("guid argument cannot be null");
            try
            {
                return _dalManager.ReadById(guid);
            }
            catch (DalBillManagerException e)
            {
                throw new BillManagerException(e.Message, e);
            }
        }

        /// <summary>
        /// Insert a bill into the datasource
        /// </summary>
        /// <param name="bill">Bill object to insert</param>
        public void Insert(Bills bill)
        {
            if (bill == null || bill.Items == null || bill.Items.Count() == 0)    //no need to call Insert operatio on DAL Layer
                return;
            try
            {
                _dalManager.Insert(bill);
            }
            catch (DalBillManagerException e)
            {
                throw new BillManagerException(e.Message, e);
            }
        }

        /// <summary>
        /// Delete a bill from datasource
        /// </summary>
        /// <param name="billId">Bill to delete</param>
        /// <returns>Number of deleted items</returns>
        public int Delete(Guid billId)
        {
            if (Guid.Empty == billId)
                throw new ArgumentException("billId argument cannot be null");
            try
            {
                return _dalManager.Delete(billId);
            }
            catch (DalBillManagerException e)
            {
                throw new BillManagerException(e.Message, e);
            }
        }

        /// <summary>
        /// Delete a list of bills from the datasource
        /// </summary>
        /// <param name="guids">Guid to delete</param>
        /// <returns>Number of delete items</returns>
        public int Delete(IEnumerable<Guid> guids)
        {
            if (guids == null || guids.Count() == 0) //no items to delete
                return 0;
            try
            {
                return _dalManager.Delete(guids);
            }
            catch (DalBillManagerException e)
            {
                throw new BillManagerException(e.Message, e);
            }
        }

        /// <summary>
        /// Update bill in the datasource
        /// </summary>
        /// <param name="billId">Bill to update</param>
        /// <param name="newBill">New bill parameters</param>
        /// <returns>Number of updated items</returns>
        public int Update(Guid billId, Bill newBill)
        {
            if (Guid.Empty == billId)
                throw new ArgumentException("billId argument cannot be null");
            try
            {
                return _dalManager.Update(billId, newBill);
            }
            catch (DalBillManagerException e)
            {
                throw new BillManagerException(e.Message, e);
            }
        }
        /// <summary>
        /// Settings for manager object
        /// </summary>
        public object [] Settings
        {
            get { return _dalManager.Settings; }
            set
            {
                try
                {
                    _dalManager.Settings = value;
                }
                catch (DalBillManagerException e)
                {
                    throw new BillManagerException(e.Message, e);
                }
            }
        }
        #endregion
    }
}