TimeZone.cs source code in C# .NET

Source code for the .NET framework in C#

                        
Code:

                         / FX-1434 / FX-1434 / 1.0 / untmp / whidbey / REDBITS / ndp / clr / src / BCL / System / TimeZone.cs / 1 / TimeZone.cs
                        

                        
                            // ==++== 
//
//   Copyright (c) Microsoft Corporation.  All rights reserved.
//
// ==--== 
/*============================================================
** 
** Class: TimeZone 
**
** 
** Purpose:
** This class is used to represent a TimeZone.  It
** has methods for converting a DateTime to UTC from local time
** and to local time from UTC and methods for getting the 
** standard name and daylight name of the time zone.
** 
** The only TimeZone that we support in version 1 is the 
** CurrentTimeZone as determined by the system timezone.
** 
**
============================================================*/
namespace System {
    using System; 
    using System.Text;
    using System.Threading; 
    using System.Collections; 
    using System.Globalization;
 
    [Serializable]
[System.Runtime.InteropServices.ComVisible(true)]
    public abstract class TimeZone {
        private static TimeZone currentTimeZone = null; 

        // Private object for locking instead of locking on a public type for SQL reliability work. 
        private static Object s_InternalSyncObject; 
        private static Object InternalSyncObject {
            get { 
                if (s_InternalSyncObject == null) {
                    Object o = new Object();
                    Interlocked.CompareExchange(ref s_InternalSyncObject, o, null);
                } 
                return s_InternalSyncObject;
            } 
        } 

 
        protected TimeZone() {
        }

        public static TimeZone CurrentTimeZone { 
            get {
                //Grabbing the cached value is required at the top of this function so that 
                //we don't incur a race condition with the ResetTimeZone method below. 
                TimeZone tz = currentTimeZone;
                if (tz == null) { 
                    lock(InternalSyncObject) {
                        if (currentTimeZone == null) {
                            currentTimeZone = new CurrentSystemTimeZone();
                        } 
                        tz = currentTimeZone;
                    } 
                } 
                return (tz);
            } 
        }

        //This method is called by CultureInfo.ClearCachedData in response to control panel
        //change events.  It must be synchronized because otherwise there is a race condition 
        //with the CurrentTimeZone property above.
        internal static void ResetTimeZone() { 
            if (currentTimeZone!=null) { 
                lock(InternalSyncObject) {
                    currentTimeZone = null; 
                }
            }
        }
 
        public abstract String StandardName {
            get; 
        } 

        public abstract String DaylightName { 
            get;
        }

        public abstract TimeSpan GetUtcOffset(DateTime time); 

        // 
        // Converts the specified datatime to the Universal time base on the current timezone 
        //
        public virtual DateTime ToUniversalTime(DateTime time) { 
            if (time.Kind == DateTimeKind.Utc) {
                return time;
            }
            long tickCount = time.Ticks - GetUtcOffset(time).Ticks; 
            if (tickCount>DateTime.MaxTicks) {
                return new DateTime(DateTime.MaxTicks, DateTimeKind.Utc); 
            } 
            if (tickCount 03:00)    10:00           -8:00   [This time doesn't actually exist, but it can be created from DateTime] 
        //      03:00               10:00           -7:00
        //      04:00               11:00           -7:00 
        //      05:00               12:00           -7:00 
        //
        //      So from 02:00 - 02:59:59, we should return the standard offset, instead of the daylight saving offset. 
        //
        // In the transition from daylight saving time to standard time,
        // if we convert local time to Universal time, we can have the
        // following (take PST as an example): 
        //      Local               Universal       UTC Offset
        //      -----               ---------       ---------- 
        //      01:00AM             08:00           -7:00 
        //      02:00 (=> 01:00)    09:00           -8:00
        //      02:00               10:00           -8:00 
        //      03:00               11:00           -8:00
        //      04:00               12:00           -8:00
        //
        //      So in this case, the 02:00 does exist after the first 2:00 rolls back to 01:00. We don't need to special case this. 
        //      But note that there are two 01:00 in the local time.
 
        // 
        // And imagine if the daylight saving offset is negative (although this does not exist in real life)
        // In the transition from standard time to daylight saving time, 
        // if we convert local time to Universal time, we can have the
        // following (take PST as an example, but the daylight saving offset is -01:00):
        //      Local               Universal       UTC Offset
        //      -----               ---------       ---------- 
        //      01:00AM             09:00           -8:00
        //      02:00 (=> 01:00)    10:00           -9:00 
        //      02:00               11:00           -9:00 
        //      03:00               12:00           -9:00
        //      04:00               13:00           -9:00 
        //      05:00               14:00           -9:00
        //
        //      So in this case, the 02:00 does exist after the first 2:00 rolls back to 01:00. We don't need to special case this.
        // 
        // In the transition from daylight saving time to standard time,
        // if we convert local time to Universal time, we can have the 
        // following (take PST as an example, daylight saving offset is -01:00): 
        //
        //      Local               Universal       UTC Offset 
        //      -----               ---------       ----------
        //      01:00AM             10:00           -9:00
        //      02:00 (=> 03:00)    11:00           -9:00
        //      03:00               11:00           -8:00 
        //      04:00               12:00           -8:00
        //      05:00               13:00           -8:00 
        //      06:00               14:00           -8:00 
        //
        //      So from 02:00 - 02:59:59, we should return the daylight saving offset, instead of the standard offset. 
        //
        internal static TimeSpan CalculateUtcOffset(DateTime time, DaylightTime daylightTimes) {
            if (daylightTimes==null) {
                return TimeSpan.Zero; 
            }
            DateTimeKind kind = time.Kind; 
            if (kind == DateTimeKind.Utc) { 
                return TimeSpan.Zero;
            } 

            DateTime startTime;
            DateTime endTime;
 
            // startTime and endTime represent the period from either the start of DST to the end and includes the
            // potentially overlapped times 
            startTime = daylightTimes.Start + daylightTimes.Delta; 
            endTime = daylightTimes.End;
 
            // For normal time zones, the ambiguous hour is the last hour of daylight saving when you wind the
            // clock back. It is theoretically possible to have a positive delta, (which would really be daylight
            // reduction time), where you would have to wind the clock back in the begnning.
            DateTime ambiguousStart; 
            DateTime ambiguousEnd;
            if (daylightTimes.Delta.Ticks > 0) { 
                ambiguousStart = endTime - daylightTimes.Delta; 
                ambiguousEnd = endTime;
            } else { 
                ambiguousStart = startTime;
                ambiguousEnd = startTime - daylightTimes.Delta;
            }
 
            Boolean isDst = false;
            if (startTime > endTime) { 
                // In southern hemisphere, the daylight saving time starts later in the year, and ends in the beginning of next year. 
                // Note, the summer in the southern hemisphere begins late in the year.
                if (time >= startTime || time < endTime) { 
                    isDst = true;
                }
            }
            else if (time>=startTime && time < endTime) { 
                // In northern hemisphere, the daylight saving time starts in the middle of the year.
                isDst = true; 
            } 

            // If this date was previously converted from a UTC date and we were able to detect that the local 
            // DateTime would be ambiguous, this data is stored in the DateTime to resolve this ambiguity.
            if (isDst && time >= ambiguousStart && time < ambiguousEnd) {
                isDst = time.IsAmbiguousDaylightSavingTime();
            } 

            if (isDst) { 
                return daylightTimes.Delta; 
            }
            return TimeSpan.Zero; 
        }
    }
}

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