操作說明：從特定日期擷取一星期的哪一日

發行項
05/09/2023

.NET 可方便判斷特定日期是一週的第幾天，並可顯示特定日期當地語系化的工作日名稱。您可以從 DayOfWeek 或 DayOfWeek 屬性取得指出對應於特定日期是星期幾的列舉值。相對地，擷取工作日名稱是一種格式化作業，可藉由呼叫格式化方法來執行，例如日期和時間值的 ToString 方法或 String.Format 方法。本文示範如何執行下列格式化作業。

擷取表示星期幾的數字

使用靜態 DateTime.Parse 或 DateTimeOffset.Parse 方法，將日期的字串表示轉換成 DateTime 或 DateTimeOffset 值。
使用 DateTime.DayOfWeek 或 DateTimeOffset.DayOfWeek 屬性，以擷取指出星期幾的 DayOfWeek 值。
若有需要，可將 DayOfWeek 值轉型 (在 C# 中) 或轉換 (在 Visual Basic 中) 成整數。

下列範例顯示代表特定日期是星期幾的整數：

using System;

public class Example
{
   public static void Main()
   {
      DateTime dateValue = new DateTime(2008, 6, 11);
      Console.WriteLine((int) dateValue.DayOfWeek);
   }
}
// The example displays the following output:
//       3

Module Example
    Public Sub Main()
        Dim dateValue As Date = #6/11/2008#
        Console.WriteLine(dateValue.DayOfWeek)
    End Sub
End Module
' The example displays the following output:
'    3

擷取工作日名稱縮寫

使用靜態 DateTime.Parse 或 DateTimeOffset.Parse 方法，將日期的字串表示轉換成 DateTime 或 DateTimeOffset 值。

您可以擷取目前文化特性或特定文化特性的工作日名稱縮寫。

若要擷取目前文化特性的工作日名稱縮寫，請呼叫日期和時間值的 DateTime.ToString(String) 或 DateTimeOffset.ToString(String) 執行個體方法，並傳遞字串 ddd 做為 format 參數。下列範例說明如何呼叫 ToString(String) 方法：

using System;

public class Example
{
   public static void Main()
   {
      DateTime dateValue = new DateTime(2008, 6, 11);
      Console.WriteLine(dateValue.ToString("ddd"));
   }
}
// The example displays the following output:
 //       Wed

Module Example
    Public Sub Main()
        Dim dateValue As Date = #6/11/2008#
        Console.WriteLine(dateValue.ToString("ddd"))
    End Sub
End Module
' The example displays the following output:
'       Wed

若要擷取特定文化特性的工作日名稱縮寫，請呼叫日期和時間值的 DateTime.ToString(String, IFormatProvider) 或 DateTimeOffset.ToString(String, IFormatProvider) 執行個體方法。傳遞字串 ddd 做為 format 參數。傳遞 CultureInfo 或 DateTimeFormatInfo 物件，其代表您想要將其工作日名稱當作 provider 參數來擷取的文化特性。下列程式碼說明如何使用代表 fr-FR 文化特性的 CultureInfo 物件，來呼叫 ToString(String, IFormatProvider) 方法：

using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      DateTime dateValue = new DateTime(2008, 6, 11);
      Console.WriteLine(dateValue.ToString("ddd",
                        new CultureInfo("fr-FR")));
   }
}
// The example displays the following output:
//       mer.

Imports System.Globalization

Module Example
    Public Sub Main()
        Dim dateValue As Date = #6/11/2008#
        Console.WriteLine(dateValue.ToString("ddd",
                          New CultureInfo("fr-FR")))
    End Sub
End Module
' The example displays the following output:
'       mer.

擷取完整的工作日名稱

使用靜態 DateTime.Parse 或 DateTimeOffset.Parse 方法，將日期的字串表示轉換成 DateTime 或 DateTimeOffset 值。

您可以擷取目前文化特性或特定文化特性的完整工作日名稱。

若要擷取目前文化特性的工作日名稱，請呼叫日期和時間值的 DateTime.ToString(String) 或 DateTimeOffset.ToString(String) 執行個體方法，並傳遞字串 dddd 做為 format 參數。下列範例說明如何呼叫 ToString(String) 方法：

using System;

public class Example
{
   public static void Main()
   {
      DateTime dateValue = new DateTime(2008, 6, 11);
      Console.WriteLine(dateValue.ToString("dddd"));
   }
}
// The example displays the following output:
//       Wednesday

Module Example
    Public Sub Main()
        Dim dateValue As Date = #6/11/2008#
        Console.WriteLine(dateValue.ToString("dddd"))
    End Sub
End Module
' The example displays the following output:
'       Wednesday

若要擷取特定文化特性的工作日名稱，請呼叫日期和時間值的 DateTime.ToString(String, IFormatProvider) 或 DateTimeOffset.ToString(String, IFormatProvider) 執行個體方法。傳遞字串 dddd 做為 format 參數。傳遞 CultureInfo 或 DateTimeFormatInfo 物件，其代表您想要將其工作日名稱當作 provider 參數來擷取的文化特性。下列程式碼說明如何使用代表 es-ES 文化特性的 CultureInfo 物件，來呼叫 ToString(String, IFormatProvider) 方法：

using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      DateTime dateValue = new DateTime(2008, 6, 11);
      Console.WriteLine(dateValue.ToString("dddd",
                        new CultureInfo("es-ES")));
   }
}
// The example displays the following output:
//       miércoles.

Imports System.Globalization

Module Example
    Public Sub Main()
        Dim dateValue As Date = #6/11/2008#
        Console.WriteLine(dateValue.ToString("dddd", _
                          New CultureInfo("es-ES")))
    End Sub
End Module
' The example displays the following output:
'       miércoles.

範例

下列範例說明對 DateTime.DayOfWeek 和 DateTimeOffset.DayOfWeek 屬性的呼叫，用於擷取代表特定日期是星期幾的數字。也包含呼叫 DateTime.ToString 和 DateTimeOffset.ToString 方法，以擷取縮寫的工作日名稱和完整的工作日名稱。

using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      string dateString = "6/11/2007";
      DateTime dateValue;
      DateTimeOffset dateOffsetValue;

      try
      {
         DateTimeFormatInfo dateTimeFormats;
         // Convert date representation to a date value
         dateValue = DateTime.Parse(dateString, CultureInfo.InvariantCulture);
         dateOffsetValue = new DateTimeOffset(dateValue,
                                      TimeZoneInfo.Local.GetUtcOffset(dateValue));

         // Convert date representation to a number indicating the day of week
         Console.WriteLine((int) dateValue.DayOfWeek);
         Console.WriteLine((int) dateOffsetValue.DayOfWeek);

         // Display abbreviated weekday name using current culture
         Console.WriteLine(dateValue.ToString("ddd"));
         Console.WriteLine(dateOffsetValue.ToString("ddd"));

         // Display full weekday name using current culture
         Console.WriteLine(dateValue.ToString("dddd"));
         Console.WriteLine(dateOffsetValue.ToString("dddd"));

         // Display abbreviated weekday name for de-DE culture
         Console.WriteLine(dateValue.ToString("ddd", new CultureInfo("de-DE")));
         Console.WriteLine(dateOffsetValue.ToString("ddd",
                                                     new CultureInfo("de-DE")));

         // Display abbreviated weekday name with de-DE DateTimeFormatInfo object
         dateTimeFormats = new CultureInfo("de-DE").DateTimeFormat;
         Console.WriteLine(dateValue.ToString("ddd", dateTimeFormats));
         Console.WriteLine(dateOffsetValue.ToString("ddd", dateTimeFormats));

         // Display full weekday name for fr-FR culture
         Console.WriteLine(dateValue.ToString("ddd", new CultureInfo("fr-FR")));
         Console.WriteLine(dateOffsetValue.ToString("ddd",
                                                    new CultureInfo("fr-FR")));

         // Display abbreviated weekday name with fr-FR DateTimeFormatInfo object
         dateTimeFormats = new CultureInfo("fr-FR").DateTimeFormat;
         Console.WriteLine(dateValue.ToString("dddd", dateTimeFormats));
         Console.WriteLine(dateOffsetValue.ToString("dddd", dateTimeFormats));
      }
      catch (FormatException)
      {
         Console.WriteLine("Unable to convert {0} to a date.", dateString);
      }
   }
}
// The example displays the following output:
//       1
//       1
//       Mon
//       Mon
//       Monday
//       Monday
//       Mo
//       Mo
//       Mo
//       Mo
//       lun.
//       lun.
//       lundi
//       lundi

Imports System.Globalization

Module Example
    Public Sub Main()
        Dim dateString As String = "6/11/2007"
        Dim dateValue As Date
        Dim dateOffsetValue As DateTimeOffset

        Try
            Dim dateTimeFormats As DateTimeFormatInfo
            ' Convert date representation to a date value
            dateValue = Date.Parse(dateString, CultureInfo.InvariantCulture)
            dateOffsetValue = New DateTimeOffset(dateValue, _
                                        TimeZoneInfo.Local.GetUtcOffset(dateValue))
            ' Convert date representation to a number indicating the day of week
            Console.WriteLine(dateValue.DayOfWeek)
            Console.WriteLine(dateOffsetValue.DayOfWeek)

            ' Display abbreviated weekday name using current culture
            Console.WriteLine(dateValue.ToString("ddd"))
            Console.WriteLine(dateOffsetValue.ToString("ddd"))

            ' Display full weekday name using current culture
            Console.WriteLine(dateValue.ToString("dddd"))
            Console.WriteLine(dateOffsetValue.ToString("dddd"))

            ' Display abbreviated weekday name for de-DE culture
            Console.WriteLine(dateValue.ToString("ddd", New CultureInfo("de-DE")))
            Console.WriteLine(dateOffsetValue.ToString("ddd", _
                                                       New CultureInfo("de-DE")))

            ' Display abbreviated weekday name with de-DE DateTimeFormatInfo object
            dateTimeFormats = New CultureInfo("de-DE").DateTimeFormat
            Console.WriteLine(dateValue.ToString("ddd", dateTimeFormats))
            Console.WriteLine(dateOffsetValue.ToString("ddd", dateTimeFormats))

            ' Display full weekday name for fr-FR culture
            Console.WriteLine(dateValue.ToString("ddd", New CultureInfo("fr-FR")))
            Console.WriteLine(dateOffsetValue.ToString("ddd", _
                                                       New CultureInfo("fr-FR")))

            ' Display abbreviated weekday name with fr-FR DateTimeFormatInfo object
            dateTimeFormats = New CultureInfo("fr-FR").DateTimeFormat
            Console.WriteLine(dateValue.ToString("dddd", dateTimeFormats))
            Console.WriteLine(dateOffsetValue.ToString("dddd", dateTimeFormats))
        Catch e As FormatException
            Console.WriteLine("Unable to convert {0} to a date.", dateString)
        End Try
    End Sub
End Module
' The example displays the following output to the console:
'       1
'       1
'       Mon
'       Mon
'       Monday
'       Monday
'       Mo
'       Mo
'       Mo
'       Mo
'       lun.
'       lun.
'       lundi
'       lundi

個別語言可能會提供功能，以複製或補充 .NET 提供的功能。例如，Visual Basic 就有包含兩個這類函式：

Weekday 會傳回指出特定日期是星期幾的數字。它會將一週第一天的序數值視為一，而 DateTime.DayOfWeek 屬性會將其視為零。
WeekdayName 會傳回目前文化特性中對應特定工作日編號的週間日名稱。

下列範例說明如何使用 Visual Basic Weekday 和 WeekdayName 函式：

Imports System.Globalization
Imports System.Threading

Module Example
    Public Sub Main()
        Dim dateValue As Date = #6/11/2008#

        ' Get weekday number using Visual Basic Weekday function
        Console.WriteLine(Weekday(dateValue))                 ' Displays 4
        ' Compare with .NET DateTime.DayOfWeek property
        Console.WriteLine(dateValue.DayOfWeek)                ' Displays 3

        ' Get weekday name using Weekday and WeekdayName functions
        Console.WriteLine(WeekdayName(Weekday(dateValue)))    ' Displays Wednesday

        ' Change culture to de-DE
        Dim originalCulture As CultureInfo = Thread.CurrentThread.CurrentCulture
        Thread.CurrentThread.CurrentCulture = New CultureInfo("de-DE")
        ' Get weekday name using Weekday and WeekdayName functions
        Console.WriteLine(WeekdayName(Weekday(dateValue)))   ' Displays Donnerstag

        ' Restore original culture
        Thread.CurrentThread.CurrentCulture = originalCulture
    End Sub
End Module

您也可以使用 DateTime.DayOfWeek 屬性傳回的值來擷取特定日期的工作日名稱。這個處理程序只需要在該屬性傳回的 DayOfWeek 值上呼叫 ToString 方法。不過，此技巧並不會產生目前文化特性的當地語系化工作日名稱，如下列範例所說明：

using System;
using System.Globalization;
using System.Threading;

public class Example
{
   public static void Main()
   {
      // Change current culture to fr-FR
      CultureInfo originalCulture = Thread.CurrentThread.CurrentCulture;
      Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-FR");

      DateTime dateValue = new DateTime(2008, 6, 11);
      // Display the DayOfWeek string representation
      Console.WriteLine(dateValue.DayOfWeek.ToString());
      // Restore original current culture
      Thread.CurrentThread.CurrentCulture = originalCulture;
   }
}
// The example displays the following output:
//       Wednesday

Imports System.Globalization
Imports System.Threading

Module Example
    Public Sub Main()
        ' Change current culture to fr-FR
        Dim originalCulture As CultureInfo = Thread.CurrentThread.CurrentCulture
        Thread.CurrentThread.CurrentCulture = New CultureInfo("fr-FR")

        Dim dateValue As Date = #6/11/2008#
        ' Display the DayOfWeek string representation
        Console.WriteLine(dateValue.DayOfWeek.ToString())
        ' Restore original current culture
        Thread.CurrentThread.CurrentCulture = originalCulture
    End Sub
End Module
' The example displays the following output:
'       Wednesday

共用方式為

操作說明：從特定日期擷取一星期的哪一日

擷取表示星期幾的數字

擷取工作日名稱縮寫

擷取完整的工作日名稱

範例

另請參閱

其他資源