列舉型別

列舉型別 (Enum) 是一種特殊格式的數值型別，從 System.Enum 繼承而來，並可為基礎的基本型別數值提供替代名稱。列舉型別具有名稱、基礎型別和一組欄位。基礎型別必須是內建帶正負號的整數 (Signed Integer) 或不帶正負號的整數 (Unsigned Integer) 型別之一 (例如 Byte、Int32 或 UInt64)。欄位為靜態的常值欄位，每一個欄位各代表一個常數。相同的數值可指定給多個欄位。發生這種情況時，必須將其中一個數值標記為反映和字串轉換的主要列舉值。

您可以將基礎型別的值指定給列舉型別，反之亦然 (Runtime 不需要轉型)。您可以建立列舉型別的執行個體，然後呼叫 System.Enum 的方法以及在列舉型別的基礎型別上定義的任何方法。但是，在需要基礎型別的執行個體時，有些語言可能不允許您將列舉型別當成參數傳遞 (反之亦然)。

下列限制適用於列舉型別：

• 不能定義自己的方法。
• 不能實作介面。
• 不能定義屬性或事件。

Flags 屬性代表一種特殊的列舉型別，稱為位元欄位 (Bit Field)。Runtime 本身無法區別傳統列舉型別和位元欄位，但是您所使用的語言或許可以區分出來。在區分時，可在位元欄位上使用位元 (Bitwise) 運算子來產生未命名的數值，但是不能用於列舉型別。列舉型別通常用於唯一的項目 (Element) 清單，例如該星期的天數、國別或區域名稱等。位元欄位通常用於可能以組合形式出現的特性或數量清單，例如 Red And Big And Fast。

下列範例顯示如何使用位元欄位和傳統列舉型別。

Imports System
Imports System.Collections

' A traditional enumeration of some root vegetables.
Public Enum SomeRootVegetables
    HorseRadish
    Radish
    Turnip
End Enum 'SomeRootVegetables


' A bit field or flag enumeration of harvesting seasons.
<Flags()> Public Enum Seasons
    None = 0
    Summer = 1
    Autumn = 2
    Winter = 4
    Spring = 8
   All = Summer Or Autumn Or Winter Or Spring
End Enum 'Seasons


' Entry point.
Public Class EnumerationSample
    
    Public Shared Sub Main()
        ' Hash table of when vegetables are available.
        Dim AvailableIn As New Hashtable()
        
        AvailableIn(SomeRootVegetables.HorseRadish) = Seasons.All
        AvailableIn(SomeRootVegetables.Radish) = Seasons.Spring
        AvailableIn(SomeRootVegetables.Turnip) = Seasons.Spring Or 
            Seasons.Autumn
        
        ' Array of the seasons, using the enumeration.
        Dim MySeasons() As Seasons = {Seasons.Summer, Seasons.Autumn, 
            Seasons.Winter, Seasons.Spring}
        
        ' Print information of what vegetables are available each season.
        Dim i As Integer
        For i = 0 To MySeasons.Length - 1
            Console.WriteLine("The following root vegetables are harvested in 
                " + MySeasons(i).ToString("G") + ":")
            Dim e As DictionaryEntry
            For Each e In  AvailableIn
                ' A bitwise comparison.
                If(CType(e.Value, Seasons) And MySeasons(i)) > 0 Then
                    Console.WriteLine(CType(e.Key, 
                        SomeRootVegetables).ToString("G"))
                End If
            Next e
        Next i
    End Sub 'Main
End Class 'EnumerationSample
[C#]
using System;
using System.Collections;

// A traditional enumeration of some root vegetables.
public enum SomeRootVegetables
{
    HorseRadish,
    Radish,
    Turnip
}

// A bit field or flag enumeration of harvesting seasons.
[Flags]
public enum Seasons
{
    None = 0,
    Summer = 1,
    Autumn = 2,
    Winter = 4,
    Spring = 8
    All = Summer | Autumn | Winter | Spring,
}

// Entry point.
public class EnumerationSample
{
    public static void Main()
    {
        // Hash table of when vegetables are available.
        Hashtable AvailableIn = new Hashtable();

        AvailableIn[SomeRootVegetables.HorseRadish] = Seasons.All;
        AvailableIn[SomeRootVegetables.Radish] = Seasons.Spring;
        AvailableIn[SomeRootVegetables.Turnip] = Seasons.Spring | 
            Seasons.Autumn;

        // Array of the seasons, using the enumeration.
        Seasons[] seasons = new Seasons[] { Seasons.Winter, Seasons.Spring, 
            Seasons.Summer, Seasons.Autumn };

        // Print information of what vegetables are available each season.
        for (int i = 0; i < seasons.Length; i++)
        {
            Console.WriteLine("\r\nThe following root vegetables are 
                harvested in " + seasons[i].ToString("G") + ":");
            foreach (DictionaryEntry e in AvailableIn)
            {
                // A bitwise comparison.
                if (((Seasons)e.Value & seasons[i]) > 0)
                    Console.WriteLine("\t" + 
                        ((SomeRootVegetables)e.Key).ToString("G"));
            }
        }
    }
}

這個程式的輸出如下。

The following root vegetables are harvested in Summer:
HorseRadish
The following root vegetables are harvested in Autumn:
Turnip
HorseRadish
The following root vegetables are harvested in Winter:
HorseRadish
The following root vegetables are harvested in Spring:
Turnip
Radish
HorseRadish

請參閱

通用型別系統 | 數值型別 | 內建數值型別 | System.ValueType |System.Enum