定義
確保某個類只有一個實體
實作方式
餓漢式加載（執行緒安全）
public sealed class Singleton
{
private static Singleton _instance = new Singleton();
//將建構式設定私有，外部不能new
private Singleton() { }
public static Singleton Instance => _instance;
}
等價于
public sealed class Singleton
{
private static Singleton _instance;
static Singleton()
{
_instance = new Singleton();
}
//將建構式設定私有，外部不能new
private Singleton() { }
public static Singleton Instance => _instance;
}
懶漢式加載
- 非執行緒安全
public sealed class Singleton
{
private static Singleton _instance;
private Singleton() { }
public static Singleton Instance => _instance = _instance ?? new Singleton();
}
- 執行緒安全
- 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寫入之前，Singleton已經初始化完成
System.Threading.Volatile.Write<Singleton>(ref _instance, temp);
}
}
return _instance;
}
}
}
- 借助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
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