.NET 設計模式實戰：單例模式線程安全實現與最佳實踐

單例模式核心定義

單例模式（Singleton Pattern）確保一個類別僅有一個實例，並提供全域存取點。核心要素：私有建構子、靜態私有實例變數、公開靜態獲取方法。

經典結構圖

線程安全的雙重檢查鎖定實現

public class Singleton
{
    private static Singleton instance;
    private static readonly object _lock = new object();

    private Singleton() { }

    public static Singleton GetInstance()
    {
        if (instance == null) // 第一次檢查，避免不必要鎖定
        {
            lock (_lock)
            {
                if (instance == null) // 第二次檢查，確保單一實例
                {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

客戶端驗證

static void Main(string[] args)
{
    Singleton singleton1 = Singleton.GetInstance();
    Singleton singleton2 = Singleton.GetInstance();
    if (singleton1 == singleton2)
    {
        Console.WriteLine("實例 singleton1 與 singleton2 相同！");
    }
    Console.ReadKey();
}

實戰案例：郵件日誌記錄器

場景：多線程環境下寫入郵件發送日誌至文件，需確保同一時間僅一個實例操作文件，避免併發衝突。

完整實現

public class EmailLog
{
    private static object _lock = new object();
    private static EmailLog _instance;
    private static object _fileLock = new object();

    private EmailLog() { }

    public static EmailLog GetInstance()
    {
        lock (_lock)
        {
            if (_instance == null)
            {
                _instance = new EmailLog();
            }
        }
        return _instance;
    }

    public void WriteEmailLog(string message)
    {
        string filePath = System.AppDomain.CurrentDomain.BaseDirectory + "mail.txt";
        StreamWriter sw = null;
        FileStream fs = null;
        lock (_fileLock)
        {
            if (!File.Exists(filePath))
            {
                fs = System.IO.File.Create(filePath);
                sw = new StreamWriter(fs, Encoding.UTF8);
            }
            else
            {
                fs = new FileStream(filePath, FileMode.Append);
                sw = new StreamWriter(fs, Encoding.UTF8);
            }
            sw.WriteLine("--------------------------------------------------------------------------");
            sw.WriteLine(message);
            sw.Flush();
            sw.Close();
        }
    }
}

客戶端調用

static void Main(string[] args)
{
    EmailLog w1 = EmailLog.GetInstance();
    w1.WriteEmailLog("發送Mail給靈動生活...");
    EmailLog w2 = EmailLog.GetInstance();
    w2.WriteEmailLog("發送Mail給James Hao...");
}

進階補充：.NET 現代寫法

.NET 4.0+ 推薦使用 Lazy<T> 實現線程安全單例，碼更簡潔：

public sealed class Singleton
{
    private static readonly Lazy _instance = 
        new Lazy(() => new Singleton());
    
    public static Singleton Instance => _instance.Value;
    private Singleton() { }
}

總結

單例模式雖簡單，但線程安全實現需謹慎。雙重檢查鎖定是經典解法，現代 .NET 建議用 Lazy<T>。本文從基礎結構、線程安全實現到實戰日誌案例完整演示，助您避開併發陷阱，正確落地單例模式。