有些編碼套路是公認的，大家都參照其編寫符合可觀賞性的代碼，那就是設計模式

現在.NETcore 默認提供了DI功能，那我想設計一個全局的引擎類，進行注入服務、解析服務、配置中間件。并且要求該引擎類全局唯一，其他地方不能進行實例化。那單例模式就派上用場了。單例模式官方定義：

確保某一個類只有一個實例，而且自行實例化并向整個系統提供這個實例，這個類成為單例類，它提供全局訪問的方法。

偽代碼實現需求

public class AAEngine { private static AAEngine aAEngine = null; private AAEngine() { } //獲取實例 public static AAEngine GetInstance() { if (aAEngine == null) { aAEngine = new AAEngine(); } return aAEngine; } //添加服務到容器 public void ConfigureService() { Console.WriteLine("添加服務到容器"); } //添加中間件到請求管道 public void ConfigureRequestPipeline() { Console.WriteLine("添加中間件到請求管道"); } //解析服務 public void Resolve<T>() where T : class { Console.WriteLine("解析服務"); }
}

在Main函數中調用

//單例模式 static void runSingleton() { var aAEngine = AAEngine.GetInstance(); aAEngine.ConfigureService(); aAEngine.ConfigureRequestPipeline(); Console.WriteLine("Oh yeah 單例模式! "); }

輸出

小結：從實例代碼中我們看到構造函數設置了級別為private，這樣可以防止外部進行new實例化，外部可以通過GetInstance方法獲取實例對象。實例代碼其實是有點瑕疵的在多線程的情況下，會違背單例的初衷，我們下面進行如何解決這個問題。

突然腦海中閃現出曾經的面試場景，餓漢式單例和懶漢式單例，或許對問題有所有幫助；

餓漢式單例

餓漢試單例是在類加載的時候就已經創建了對象。代碼如下

public class AAEngine1 { private static AAEngine1 aAEngine = new AAEngine1(); private AAEngine1() { } //獲取實例 public static AAEngine1 GetInstance() { return aAEngine; }
//添加服務到容器 public void ConfigureService() { Console.WriteLine("添加服務到容器"); } //添加中間件到請求管道 public void ConfigureRequestPipeline() { Console.WriteLine("添加中間件到請求管道"); } //解析服務 public void Resolve<T>() where T : class { Console.WriteLine("解析服務"); } }小結：在類被加載時，靜態變量aAEngine會被初始化，AAEngine1類的唯一實例將會創建，則多線程并發的場景下，也可確保單例對象的唯一性；那什么是懶漢式單例呢？其實最上面的AAEngine就是懶漢式單例，在多線程并發的場景下懶漢式單例有問題，如何解決 答案是通過鎖的方式。

懶漢式單例+線程鎖

懶漢式單例有延遲Lazy的思想，只有在需要的時候才去加載實例。在多線程并發的場景下我們使用雙重檢查鎖定（Double-Check Locking）。完成代碼如下：

private static AAEngine aAEngine = null; private static object lockObj=new object(); private AAEngine() { } //獲取實例 public static AAEngine GetInstance() { //第一重驗證 if (aAEngine == null) { lock (lockObj) { //第二重驗證 if (aAEngine==null) { aAEngine = new AAEngine(); } } } return aAEngine; }

單例模式在開源Nop項目中實踐

為了配合你沒有閱讀過Nop項目源碼，我會把涉及到單例的幾個類源碼貼出來。主要設計到3個類Singleton、IEngine、EngineContext。

Singleton類

public class Singleton<T> : BaseSingleton { private static T instance;
/// <summary> /// The singleton instance for the specified type T. Only one instance (at the time) of this object for each type of T. /// </summary> public static T Instance { get => instance; set { instance = value; AllSingletons[typeof(T)] = value; } } }

IEngine類

public interface IEngine { /// <summary> /// 添加配置服務 Add and configure services /// </summary> /// <param name="services">Collection of service descriptors</param> /// <param name="configuration">Configuration of the application</param> /// <param name="nopConfig">Nop configuration parameters</param> /// <returns>Service provider</returns> IServiceProvider ConfigureServices(IServiceCollection services, IConfiguration configuration, NopConfig nopConfig);
/// <summary> /// 配置請求管道 Configure HTTP request pipeline /// </summary> /// <param name="application">Builder for configuring an application's request pipeline</param> void ConfigureRequestPipeline(IApplicationBuilder application);
/// <summary> /// 解析服務 Resolve dependency /// </summary> /// <typeparam name="T">Type of resolved service</typeparam> /// <returns>Resolved service</returns> T Resolve<T>() where T : class;
/// <summary> /// 解析服務 Resolve dependency /// </summary> /// <param name="type">Type of resolved service</param> /// <returns>Resolved service</returns> object Resolve(Type type);
/// <summary> /// 解析所有服務Resolve dependencies /// </summary> /// <typeparam name="T">Type of resolved services</typeparam> /// <returns>Collection of resolved services</returns> IEnumerable<T> ResolveAll<T>();
/// <summary> /// Resolve unregistered service /// </summary> /// <param name="type">Type of service</param> /// <returns>Resolved service</returns> object ResolveUnregistered(Type type); }

EngineContext 引擎上下文類

#region Methods
/// <summary> /// Create a static instance of the Nop engine. /// </summary> [MethodImpl(MethodImplOptions.Synchronized)] public static IEngine Create() { //create NopEngine as engine return Singleton<IEngine>.Instance ?? (Singleton<IEngine>.Instance = new NopEngine()); }
/// <summary> /// Sets the static engine instance to the supplied engine. Use this method to supply your own engine implementation. /// </summary> /// <param name="engine">The engine to use.</param> /// <remarks>Only use this method if you know what you're doing.</remarks> public static void Replace(IEngine engine) { Singleton<IEngine>.Instance = engine; } #endregion
#region Properties
/// <summary> /// Gets the singleton Nop engine used to access Nop services. /// </summary> public static IEngine Current { get { if (Singleton<IEngine>.Instance == null) { Create(); }
return Singleton<IEngine>.Instance; } }
#endregion }

從nop的源碼中我們發現，他使用的懶漢式單例（含雙重檢查鎖定），外部訪問IEngine實例是通過EngineContext上下文來訪問的。在創建IEngine實例方法create（）時，使用時的 [MethodImpl(MethodImplOptions.Synchronized)]特性，表示create方法只能由一個線程執行，類似lock鎖。

如何使用，代碼如下

//create engine and configure service provider var engine = EngineContext.Create(); var serviceProvider = engine.ConfigureServices(services, configuration, nopConfig); 。。。。。。。。。。。。 EngineContext.Current.Resolve<IScheduleTaskService>();

nop封裝的優秀的代碼，也收錄到我的開源項目中了，喜歡可以star下?https://github.com/ChengLab/AAFrameWork

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