如何：往返日期和时间值

项目
08/12/2011

在许多应用程序中，日期和时间值可用于明确地标识单个时间点。本主题演示如何将 DateTime 值、DateTimeOffset 值以及日期和时间值与时区信息一起进行保存和还原，以便还原值可以标识与保存值相同的时间。

保存/还原 DateTime 值

通过使用“o”格式说明符调用 DateTime.ToString(String) 方法，将 DateTime 值转换为相应的字符串表示形式。
将 DateTime 值的字符串表示形式保存到文件中，或跨越进程、应用程序域或计算机边界传递它。
检索表示 DateTime 值的字符串。
调用 DateTime.Parse(String, IFormatProvider, DateTimeStyles) 方法，并将 DateTimeStyles.RoundtripKind 作为 styles 参数的值传递。

下面的示例说明如何保存/还原 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.

保存/还原 DateTime 值时，此方法可成功地保留所有本地时间和世界时间的时间。例如，如果本地 DateTime 值保存在位于美国太平洋标准时区的系统上并在位于美国中央标准时区的系统上还原，则还原的日期和时间将比原始时间晚两个小时，这反映了两个时区之间的时差。但是，此方法对于未明确的时间不一定准确。其 Kind 属性为 Unspecified 的所有 DateTime 值都会作为本地时间进行处理。如果不是这样，DateTime 将不会成功标识正确的时间点。此限制的解决方法是在保存和还原操作中将日期和时间值与对应的时区紧密耦合在一起。

保存/还原 DateTimeOffset 值

通过使用“o”格式说明符调用 DateTimeOffset.ToString(String) 方法，将 DateTimeOffset 值转换为相应的字符串表示形式。
将 DateTimeOffset 值的字符串表示形式保存到文件中，或跨越进程、应用程序域或计算机边界传递它。
检索表示 DateTimeOffset 值的字符串。
调用 DateTimeOffset.Parse(String, IFormatProvider, DateTimeStyles) 方法，并将 DateTimeStyles.RoundtripKind 作为 styles 参数的值传递。

下面的示例说明如何保存/还原 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.

此方法始终可以将 DateTimeOffset 值明确标识为单个时间点。然后，可以通过调用 DateTimeOffset.ToUniversalTime 方法将该值转换为协调世界时 (UTC)，或通过调用 DateTimeOffset.ToOffset 或 TimeZoneInfo.ConvertTime(DateTimeOffset, TimeZoneInfo) 方法将该值转换为某个特定时区的时间。此方法的主要限制是日期和时间算法，当对表示某个特定时区中的时间的 DateTimeOffset 值执行日期和时间算法时，可能不会得到对应于该时区的准确结果。这是因为在实例化 DateTimeOffset 值时，该值会与其时区解除关联。因此，当执行日期和时间计算时，不会再应用该时区的调整规则。通过定义包括日期和时间值以及其相应时区的自定义类型，可以解决此问题。

保存/还原带有时区的日期和时间值

定义带有两个字段的类或结构。第一个字段是 DateTime 或 DateTimeOffset 对象，第二个字段是 TimeZoneInfo 对象。下面的示例是此类型的一个简单版本。

<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;
      }
   }
}

使用 SerializableAttribute 特性标记该类。
使用 BinaryFormatter.Serialize 方法序列化对象。
使用 Deserialize 方法还原对象。
将反序列化对象强制转换（在 C# 中）或转换（在 Visual Basic 中）为相应类型的对象。

下面的示例演示如何保存/还原存储日期和时间以及时区信息的对象。

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

在保存和还原日期和时间值的前后，此方法都应始终明确地反映正确的时间点，前提是日期和时间与时区的组合对象的实现不允许日期值与时间值不同步。

编译代码

这些示例要求：

使用 C# using 语句或 Visual Basic Imports 语句导入下列命名空间：
- System（仅用于 C#）
- System.Globalization.
- System.IO.
- System.Runtime.Serialization.
- System.Runtime.Serialization.Formatters.Binary.
对 System.Core.dll 的引用。
除 DateInTimeZone 类之外的每个代码示例，应包含在一个类或 Visual Basic 模型中，并包装在方法中，然后从 Main 方法调用。

请参见

概念

执行格式设置操作

在 DateTime、DateTimeOffset 和 TimeZoneInfo 之间进行选择

标准日期和时间格式字符串

通过

如何：往返日期和时间值

保存/还原 DateTime 值

保存/还原 DateTimeOffset 值

保存/还原带有时区的日期和时间值

编译代码

请参见

概念

其他资源