【設計模式】單例模式

定義

確保某個類只有一個實例

實現方式

餓漢式加載（線程安全）

public sealed class Singleton
{
    private static Singleton _instance = new Singleton();
    //將構造函數設置私有，外部不能new
    private Singleton() { }
    public static Singleton Instance => _instance;
}

等價於

public sealed class Singleton
{
    private static Singleton _instance;
    static Singleton()
    {
        _instance = new Singleton();
    }
    //將構造函數設置私有，外部不能new 
    private Singleton() { }
    public static Singleton Instance => _instance;
}

懶漢式加載

• 非線程安全

public sealed class Singleton
{
    private static Singleton _instance;
    private Singleton() { }
    public static Singleton Instance => _instance = _instance ?? new Singleton();
}

• 線程安全

1. Double Check

public sealed class Singleton
{
    private static readonly object _lock = new object();
    private static Singleton _instance;
    private Singleton()
    {
        Console.WriteLine("Singleton Constructor");
    }
    public static Singleton Instance
    {
        get
        {
            /// 避免走內核代碼
            if (_instance != null) return _instance;

            lock (_lock)
            {
                if (_instance == null)
                {
                    var temp = new Singleton();
                    //確保_instance寫入以前，Singleton已經初始化完成
                    System.Threading.Volatile.Write<Singleton>(ref _instance, temp);
                }
            }
            return _instance;
        }
    }
}

2. 藉助Lazy

public sealed class Singleton
{
    private static Lazy<Singleton> _instance = new Lazy<Singleton>(() => new Singleton(), true);
    private Singleton()
    {
        Console.WriteLine("Singleton Constructor");
    }
    public static Singleton Instance => _instance.Value;
}

示例代碼 - github