HOW TO：反覆存取日期和時間值

發行項
08/21/2008

更新：2007 年 11 月

在許多應用程式中，日期和時間值是用來明確識別單一時間點。本主題示範如何儲存和還原 DateTime 值、DateTimeOffset 值以及具備時區資訊的日期和時間值，如此一來，還原的值就可將相同的時間識別為儲存的值。

若要反覆存取 DateTime 值

呼叫 DateTime.ToString(String) 方法加上 "o" 格式規範，將 DateTime 值轉換成其字串表示。
將 DateTime 值的字串表示儲存到檔案，或跨程序、應用程式定義域或電腦界限傳遞。
擷取表示 DateTime 值的字串。
呼叫 DateTime.Parse(String, IFormatProvider, DateTimeStyles) 方法，並將 DateTimeStyles.RoundtripKind 當做 styles 參數的值傳遞。

下列範例示範如何反覆存取 DateTime 值。

Const fileName As String = ".\DateFile.txt"

Dim outFile As New StreamWriter(fileName)

' Save DateTime value.
Dim dateToSave As Date = DateTime.SpecifyKind(#06/12/2008 6:45:15 PM#, _
                                              DateTimeKind.Local)
Dim dateString As String = dateToSave.ToString("o")      
Console.WriteLine("Converted {0} ({1}) to {2}.", dateToSave.ToString(), _
                  dateToSave.Kind.ToString(), dateString)      
outFile.WriteLine(dateString)
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName)
outFile.Close()   

' Restore DateTime value.
Dim restoredDate As Date

Dim inFile As New StreamReader(fileName)
dateString = inFile.ReadLine()
inFile.Close()
restoredDate = DateTime.Parse(dateString, Nothing, DateTimeStyles.RoundTripKind)
Console.WriteLine("Read {0} ({2}) from {1}.", restoredDate.ToString(), _
                  fileName, restoredDAte.Kind.ToString())
' The example displays the following output:
'    Converted 6/12/2008 6:45:15 PM (Local) to 2008-06-12T18:45:15.0000000-05:00.
'    Wrote 2008-06-12T18:45:15.0000000-05:00 to .\DateFile.txt.
'    Read 6/12/2008 6:45:15 PM (Local) from .\DateFile.txt.

const string fileName = @".\DateFile.txt";

StreamWriter outFile = new StreamWriter(fileName);

// Save DateTime value.
DateTime dateToSave = DateTime.SpecifyKind(new DateTime(2008, 6, 12, 18, 45, 15), 
                                           DateTimeKind.Local);
string dateString = dateToSave.ToString("o");      
Console.WriteLine("Converted {0} ({1}) to {2}.", 
                  dateToSave.ToString(), 
                  dateToSave.Kind.ToString(), 
                  dateString);      
outFile.WriteLine(dateString);
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName);
outFile.Close();

// Restore DateTime value.
DateTime restoredDate;

StreamReader inFile = new StreamReader(fileName);
dateString = inFile.ReadLine();
inFile.Close();
restoredDate = DateTime.Parse(dateString, null, DateTimeStyles.RoundtripKind);
Console.WriteLine("Read {0} ({2}) from {1}.", restoredDate.ToString(), 
                                              fileName, 
                                              restoredDate.Kind.ToString());
// The example displays the following output:
//    Converted 6/12/2008 6:45:15 PM (Local) to 2008-06-12T18:45:15.0000000-05:00.
//    Wrote 2008-06-12T18:45:15.0000000-05:00 to .\DateFile.txt.
//    Read 6/12/2008 6:45:15 PM (Local) from .\DateFile.txt.

反覆存取 DateTime 值時，此方式能成功保存所有本地和國際標準時間的時間。例如，如果本地 DateTime 值儲存在美國太平洋標準時間系統上，而在美國中央標準時間系統上還原，還原的日期和時間就會是原始時間再加上兩小時，反映出兩個時區之間的時差。不過，這種方式對未指定的時間不一定精確。Kind 屬性為 Unspecified 的所有 DateTime 值都會視為是本地時間。若非如此，DateTime 將無法成功識別正確的時間點。為解決此限制，在進行儲存和還原作業時，可以將日期和時間值與其時區緊密結合在一起。

若要反覆存取 DateTimeOffset 值

呼叫 DateTimeOffset.ToString(String) 方法加上 "o" 格式規範，將 DateTimeOffset 值轉換成其字串表示。
將 DateTimeOffset 值的字串表示儲存到檔案，或跨程序、應用程式定義域或電腦界限傳遞。
擷取表示 DateTimeOffset 值的字串。
呼叫 DateTimeOffset.Parse(String, IFormatProvider, DateTimeStyles) 方法，並將 DateTimeStyles.RoundtripKind 當做 styles 參數的值傳遞。

下列範例示範如何反覆存取 DateTimeOffset 值。

Const fileName As String = ".\DateOff.txt"

Dim outFile As New StreamWriter(fileName)

' Save DateTime value.
Dim dateToSave As New DateTimeOffset(2008, 6, 12, 18, 45, 15, _
                                     New TimeSpan(7, 0, 0))
Dim dateString As String = dateToSave.ToString("o")      
Console.WriteLine("Converted {0} to {1}.", dateToSave.ToString(), dateString)      
outFile.WriteLine(dateString)
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName)
outFile.Close()   

' Restore DateTime value.
Dim restoredDateOff As DateTimeOffset

Dim inFile As New StreamReader(fileName)
dateString = inFile.ReadLine()
inFile.Close()
restoredDateOff = DateTimeOffset.Parse(dateString, Nothing, DateTimeStyles.RoundTripKind)
Console.WriteLine("Read {0} from {1}.", restoredDateOff.ToString(), fileName)
' The example displays the following output:
'    Converted 6/12/2008 6:45:15 PM +07:00 to 2008-06-12T18:45:15.0000000+07:00.
'    Wrote 2008-06-12T18:45:15.0000000+07:00 to .\DateOff.txt.
'    Read 6/12/2008 6:45:15 PM +07:00 from .\DateOff.txt.

const string fileName = @".\DateOff.txt";

StreamWriter outFile = new StreamWriter(fileName);

// Save DateTime value.
DateTimeOffset dateToSave = new DateTimeOffset(2008, 6, 12, 18, 45, 15, 
                                               new TimeSpan(7, 0, 0));
string dateString = dateToSave.ToString("o");      
Console.WriteLine("Converted {0} to {1}.", dateToSave.ToString(), 
                  dateString);      
outFile.WriteLine(dateString);
Console.WriteLine("Wrote {0} to {1}.", dateString, fileName);
outFile.Close();

// Restore DateTime value.
DateTimeOffset restoredDateOff;

StreamReader inFile = new StreamReader(fileName);
dateString = inFile.ReadLine();
inFile.Close();
restoredDateOff = DateTimeOffset.Parse(dateString, null, 
                                       DateTimeStyles.RoundtripKind);
Console.WriteLine("Read {0} from {1}.", restoredDateOff.ToString(), 
                  fileName);
// The example displays the following output:
//    Converted 6/12/2008 6:45:15 PM +07:00 to 2008-06-12T18:45:15.0000000+07:00.
//    Wrote 2008-06-12T18:45:15.0000000+07:00 to .\DateOff.txt.
//    Read 6/12/2008 6:45:15 PM +07:00 from .\DateOff.txt.

此方式永遠能將 DateTimeOffset 值明確識別為單一時間點。此值接著可以藉由呼叫 DateTimeOffset.ToUniversalTime 方法轉換為 Coordinated Universal Time (UTC)，或可以藉由呼叫 DateTimeOffset.ToOffset or TimeZoneInfo.ConvertTime(DateTimeOffset, TimeZoneInfo) 方法轉換為特定時區的時間。此方式的主要限制在於，如果在表示特定時區時間的 DateTimeOffset 值上執行日期和時間運算，可能不會產生該時區的精確結果。這是因為當 DateTimeOffset 值執行個體化時，會與其時區解除關聯。因此，當您執行日期和時間計算時，就無法再套用該時區的調整規則。要解決此問題，您可以定義自訂型別，將日期和時間值以及其伴隨的時區包含在內。

若要反覆存取具有時區的日期和時間值

定義有兩個欄位的類別或結構。第一個欄位是 DateTime 或 DateTimeOffset 物件，而第二個欄位是 TimeZoneInfo 物件。下列範例是這種型別的簡單版本。

<Serializable> Public Class DateInTimeZone
   Private tz As TimeZoneInfo
   Private thisDate As DateTimeOffset

   Public Sub New()
   End Sub

   Public Sub New(date1 As DateTimeOffset, timeZone As TimeZoneInfo)
      If timeZone Is Nothing Then 
         Throw New ArgumentNullException("The time zone cannot be null.")
      End If
      Me.thisDate = date1
      Me.tz = timeZone
   End Sub

   Public Property DateAndTime As DateTimeOffset
      Get
         Return Me.thisDate
      End Get
      Set
         If Value.Offset <> Me.tz.GetUtcOffset(Value) Then
            Me.thisDate = TimeZoneInfo.ConvertTime(Value, tz)
         Else
            Me.thisDate = Value
         End If
      End Set
   End Property

   Public ReadOnly Property TimeZone As TimeZoneInfo
      Get
         Return tz
      End Get
   End Property
End Class

[Serializable] public class DateInTimeZone
{
   private TimeZoneInfo tz;
   private DateTimeOffset thisDate;

   public DateInTimeZone() {}

   public DateInTimeZone(DateTimeOffset date, TimeZoneInfo timeZone)
   {
      if (timeZone == null) 
         throw new ArgumentNullException("The time zone cannot be null.");

      this.thisDate = date;
      this.tz = timeZone;
   }

   public DateTimeOffset DateAndTime
   {
      get { 
         return this.thisDate;
      }
      set {
         if (value.Offset != this.tz.GetUtcOffset(value)) 
            this.thisDate = TimeZoneInfo.ConvertTime(value, tz);
         else
            this.thisDate = value;
      }
   }

   public TimeZoneInfo TimeZone
   {
      get {
         return this.tz;
      }
   }
}

以 SerializableAttribute 屬性標記類別。
使用 BinaryFormatter.Serialize 方法序列化物件。
使用 Deserialize 方法還原物件。
將還原序列化的物件轉型 (C#) 或轉換 (Visual Basic) 為適當型別的物件。

下列範例示範如何反覆存取同時儲存日期和時間以及時區資訊的物件。

Const fileName As String = ".\DateWithTz.dat"

Dim tempDate As Date = #9/3/2008 7:00:00 PM#
Dim tempTz As TimeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time")
Dim dateWithTz As New DateInTimeZone(New DateTimeOffset(tempDate, _
                                         tempTz.GetUtcOffset(tempDate)), _
                                     tempTz)

' Store DateInTimeZone value to a file
Dim outFile As New FileStream(fileName, FileMode.Create)
Try
   Dim formatter As New BinaryFormatter()
   formatter.Serialize(outFile, dateWithTz)
   Console.WriteLine("Saving {0} {1} to {2}", dateWithTz.DateAndTime, _
           IIf(dateWithTz.TimeZone.IsDaylightSavingTime(dateWithTz.DateAndTime), _
               dateWithTz.TimeZone.DaylightName, dateWithTz.TimeZone.DaylightName), _
           fileName)
Catch e As SerializationException
   Console.WriteLine("Unable to serialize time data to {0}.", fileName)
Finally   
   outFile.Close()
End Try

' Retrieve DateInTimeZone value
If File.Exists(fileName) Then
   Dim inFile As New FileStream(fileName, FileMode.Open)
   Dim dateWithTz2 As New DateInTimeZone()
   Try
      Dim formatter As New BinaryFormatter()
      dateWithTz2 = DirectCast(formatter.Deserialize(inFile), DateInTimeZone)         
      Console.WriteLine("Restored {0} {1} from {2}", dateWithTz2.DateAndTime, _
                        IIf(dateWithTz2.TimeZone.IsDaylightSavingTime(dateWithTz2.DateAndTime), _
                        dateWithTz2.TimeZone.DaylightName, dateWithTz2.TimeZone.DaylightName), _
                        fileName)
   Catch e As SerializationException
      Console.WriteLine("Unable to retrieve date and time information from {0}", _
                        fileName)
   Finally            
      inFile.Close
   End Try
End If
' This example displays the following output to the console:
'    Saving 9/3/2008 7:00:00 PM -05:00 Central Daylight Time to .\DateWithTz.dat
'    Restored 9/3/2008 7:00:00 PM -05:00 Central Daylight Time from .\DateWithTz.dat

const string fileName = @".\DateWithTz.dat";

DateTime tempDate = new DateTime(2008, 9, 3, 19, 0, 0);
TimeZoneInfo tempTz = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");
DateInTimeZone dateWithTz = new DateInTimeZone(new DateTimeOffset(tempDate, 
                                tempTz.GetUtcOffset(tempDate)), 
                                tempTz);

// Store DateInTimeZone value to a file
FileStream outFile = new FileStream(fileName, FileMode.Create);
try
{
   BinaryFormatter formatter = new BinaryFormatter();
   formatter.Serialize(outFile, dateWithTz);
   Console.WriteLine("Saving {0} {1} to {2}", dateWithTz.DateAndTime, 
                     dateWithTz.TimeZone.IsDaylightSavingTime(dateWithTz.DateAndTime) ? 
                     dateWithTz.TimeZone.DaylightName : dateWithTz.TimeZone.DaylightName, 
                     fileName);
}
catch (SerializationException)
{
   Console.WriteLine("Unable to serialize time data to {0}.", fileName);
}
finally
{         
   outFile.Close();
}

// Retrieve DateInTimeZone value
if (File.Exists(fileName))
{
   FileStream inFile = new FileStream(fileName, FileMode.Open);
   DateInTimeZone dateWithTz2 = new DateInTimeZone();
   try
   {
      BinaryFormatter formatter = new BinaryFormatter();
      dateWithTz2 = formatter.Deserialize(inFile) as DateInTimeZone;   
      Console.WriteLine("Restored {0} {1} from {2}", dateWithTz2.DateAndTime, 
                        dateWithTz2.TimeZone.IsDaylightSavingTime(dateWithTz2.DateAndTime) ? 
                        dateWithTz2.TimeZone.DaylightName : dateWithTz2.TimeZone.DaylightName, 
                        fileName);
   }
   catch (SerializationException)
   {
      Console.WriteLine("Unable to retrieve date and time information from {0}", 
                        fileName);
   }
   finally
   {
      inFile.Close();
   }
}
// This example displays the following output to the console:
//    Saving 9/3/2008 7:00:00 PM -05:00 Central Daylight Time to .\DateWithTz.dat
//    Restored 9/3/2008 7:00:00 PM -05:00 Central Daylight Time from .\DateWithTz.dat

此方式應能永遠明確反映出正確的時間點，不論是在儲存和還原之前或之後，但前提是日期和時間以及時區合併物件的實作不允許日期值與時區值變成不同步。

編譯程式碼

這些範例需要：

使用 C# using 陳述式或 Visual Basic Imports 陳述式匯入下列命名空間：
- System (僅限 C#)。
- System.Globalization.
- System.IO.
- System.Runtime.Serialization.
- System.Runtime.Serialization.Formatters.Binary.
System.Core.dll 的參考。
每個程式碼範例 (除 DateInTimeZone 類別外) 都應包含在類別或 Visual Basic 模組中、包裝在方法中，以及從 Main 方法呼叫。

請參閱

概念

格式設定 HOW TO 主題

在 DateTime、DateTimeOffset 和 TimeZoneInfo 之間選擇

標準日期和時間格式字串

共用方式為

HOW TO：反覆存取日期和時間值

若要反覆存取 DateTime 值

若要反覆存取 DateTimeOffset 值

若要反覆存取具有時區的日期和時間值

編譯程式碼

請參閱

概念

其他資源