Building a Web Message Board using Visual Studio 2008, Part I - The Basic Message Board

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Collections.Generic;
using MessageBoard.Web.Properties;
using System.Runtime.Serialization;

namespace MessageBoard.Web
{
    /// <summary>
    /// This class has all teh helper functions to detect and save time zone values
    /// </summary>
    public static class TimeZoneUtility
    {
        /// <summary>
        /// Gets a list of time zones which have the specified UTC offset in minutes. If the string is invalid
        /// all time zones are returned.
        /// </summary>
        /// <param name="jsUtcOffset">The Utc offset from JavaScript ontained from the Request</param>
        /// <returns>List of time zones</returns>
        public static IEnumerable<TimeZoneInfo> GetTimeZonesFromUtcOffset(string jsUtcOffset)
        {
            int timeZoneOffset = 0;

            return (Int32.TryParse(jsUtcOffset, out timeZoneOffset)) ?
                GetTimeZonesFromUtcOffset(timeZoneOffset) :
                TimeZoneInfo.GetSystemTimeZones();
        }

        /// <summary>
        /// Returns timezone offset from specified offset obtained from JavaScript
        /// </summary>
        /// <param name="timeZoneOffset">Offset obtained from javaScript's getTimezoneOffset method</param>
        /// <returns>List of time zones</returns>
        public static IEnumerable<TimeZoneInfo> GetTimeZonesFromUtcOffset(int timeZoneOffset)
        {
            TimeSpan jsOffsetTimeSpan = new TimeSpan(0, -timeZoneOffset, 0);

            //UTC offset is in range from -14 to +14 hours
            //TimeSpan.Duration gives us the absolute value
            if (jsOffsetTimeSpan.Duration().TotalHours > 14)
                return TimeZoneInfo.GetSystemTimeZones();

            return from tz in TimeZoneInfo.GetSystemTimeZones()
                   where tz.BaseUtcOffset.Equals(jsOffsetTimeSpan)
                   select tz;
        }
    
        /// <summary>
        /// Returns TimeZoneInfo given the time zone id. If the timezone is invalid
        /// it returns the local timzezone
        /// </summary>
        /// <param name="id">The timezone id specified as string</param>
        /// <returns>The time zone info</returns>
        public static TimeZoneInfo GetTimeZoneFromId(string id)
        {
            if (String.IsNullOrEmpty(id))
                return TimeZoneInfo.Local;

            TimeZoneInfo info;

            try
            {
                info = TimeZoneInfo.FindSystemTimeZoneById(id);
            }
            catch (TimeZoneNotFoundException)
            {
                info = TimeZoneInfo.Local;
            }

            return info;
        }

        private static readonly string CookieName = "__TimeZoneInfo";

        /// <summary>
        /// Saves the time zone info in a cookie
        /// </summary>
        /// <param name="info">The time zone info to save in a cookie</param>
        public static void SaveTimeZoneInfoInCookie(TimeZoneInfo info)
        {
            HttpContext context = HttpContext.Current;

            if (context == null)
                throw new InvalidOperationException(Resources.NullHttpContext);
            
            HttpCookie cookie = new HttpCookie(CookieName, info.Id);
            cookie.Expires = DateTime.Now.AddYears(1); //Expire after a year
            context.Response.AppendCookie(cookie);
        }

        /// <summary>
        /// Uses the current HttpContext to obtain a TimzeZoneInfo value stored in a cookie.
        /// If no such TimezonInfo is found or the cookie data is invalid this function returns null
        /// </summary>
        /// <returns>The TimeZoneInfo from the cookie or null otherwise</returns>
        public static TimeZoneInfo GetTimeZoneInfoFromCookie()
        {
            HttpContext context = HttpContext.Current;

            if (context == null)
                throw new InvalidOperationException(Resources.NullHttpContext);

            HttpCookie cookie = context.Request.Cookies[CookieName];

            TimeZoneInfo info = TimeZoneInfo.Utc;

            if (cookie == null || String.IsNullOrEmpty(cookie.Value))
                return info;

            try
            {
                info = TimeZoneInfo.FindSystemTimeZoneById(cookie.Value);

            }
            catch (TimeZoneNotFoundException )
            {
                //It's ok just return null               
            }

            return info;
        }

        public static DateTime ConvertToCurrentTimeZone(DateTime dateTime)
        {
            return TimeZoneInfo.ConvertTimeFromUtc(dateTime, TimeZoneUtility.GetRequestTimeZone());
        }

        private static object _timeZoneKey = new object();

        /// <summary>
        /// Gets the current time zone of the Request
        /// </summary>
        /// <returns>The timezone value</returns>
        public static TimeZoneInfo GetRequestTimeZone()
        {
            TimeZoneInfo info = HttpContext.Current.Items[_timeZoneKey] as TimeZoneInfo;

            if (info == null)
            {
                info = GetTimeZoneInfoFromCookie();
                HttpContext.Current.Items[_timeZoneKey] = info;
            }

            return info;
        }
    }
}