C# 列舉型別：命名常數、底層型別、Flags 屬性、位元運算與最佳實務

編輯：關於C語言

列舉本質：具名常數集合

列舉讓數值擁有語意名稱，提升可讀性與型別安全：

enum WeekDay
{
    Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday
}

WeekDay day = WeekDay.Tuesday;  // 型別安全，不可指派任意整數

底層型別與預設值

預設底層型別：int
第一個成員 = 0，後續遞增 1
可指定底層型別：byte、short、int、long（及無號對應）

enum Priority : byte  // 節省記憶體
{
    Low = 1,
    Medium = 2,
    High = 3
}

手動指定值與重複值

enum HttpStatus
{
    OK = 200,
    Created = 201,
    NoContent = 204,
    
    MovedPermanently = 301,
    Found = 302,           // 302
    Redirect = 302,        // 允許重複值（別名）
    
    BadRequest = 400,
    Unauthorized = 401,
    NotFound = 404
}

HttpStatus.Found == HttpStatus.Redirect 為 true。

Flags 屬性：位元旗標組合

[Flags]
enum FileAccess
{
    None = 0,
    Read = 1,      // 2⁰
    Write = 2,     // 2¹
    Execute = 4,   // 2²
    ReadWrite = Read | Write  // 3
}

使用：

FileAccess access = FileAccess.Read | FileAccess.Write;
bool canRead = (access & FileAccess.Read) != 0;  // true
access &= ~FileAccess.Write;  // 移除 Write

⚠️ Enum.HasFlag() 方便但會裝箱，效能敏感場合用位元運算。

常用靜態方法

方法	用途	範例
`Enum.GetValues()`	取得所有值陣列	`foreach (WeekDay d in Enum.GetValues(typeof(WeekDay)))`
`Enum.GetNames()`	取得所有名稱陣列	`string[] names = Enum.GetNames(typeof(WeekDay));`
`Enum.Parse()`	字串轉列舉	`WeekDay d = (WeekDay)Enum.Parse(typeof(WeekDay), "Monday");`
`Enum.TryParse()`	安全轉換	`Enum.TryParse("Tuesday", out WeekDay d);`
`Enum.IsDefined()`	驗證值是否定義	`Enum.IsDefined(typeof(WeekDay), 7); // false`
`Enum.GetName()`	值轉名稱	`string name = Enum.GetName(typeof(WeekDay), 1); // "Monday"`

進階：C# 7.0+ 模式比對

switch (day)
{
    case WeekDay.Saturday:
    case WeekDay.Sunday:
        Console.WriteLine("週末");
        break;
    case WeekDay d when d >= WeekDay.Monday && d <= WeekDay.Friday:
        Console.WriteLine("平日");
        break;
}