（3）Asp.Net Core 服務生命周期

1.前言

在ConfigureServices方法中的容器註冊每個應用程式的服務，Asp.Core都可以為每個應用程式提供三種服務生命周期：
●Transient（暫時）：每次請求都會創建一個新的實例。這種生命周期最適合輕量級，無狀態服務。
●Scoped（作用域）：在同一個作用域內只初始化一個實例 ，可以理解為每一個請求只創建一個實例，同一個請求會在一個作用域內。
●Singleton（單例）：整個應用程式生命周期以內只創建一個實例，後續每個請求都使用相同的實例。如果應用程式需要單例行為，建議讓服務容器管理服務的生命周期，而不是在自己的類中實現單例模式。

2.服務生命周期與註冊選項案例演示

為了演示生命周期和註冊選項之間的差異，請考慮以下介面，將任務表示為具有唯一標識符 OperationId 的操作。根據以下介面配置操作服務的生命周期的方式，容器在類請求時提供相同或不同的服務實例：

public interface IOperation
{
    Guid OperationId { get; }
}
public interface IOperationTransient : IOperation
{
}
public interface IOperationScoped : IOperation
{
}
public interface IOperationSingleton : IOperation
{
}
public interface IOperationSingletonInstance : IOperation
{
}

上面四種服務介面在 Operation 類中實現。調用Operation類時將自動生成一個GUID，下麵是Operation類的實現：

public class Operation : IOperationTransient, IOperationScoped, IOperationSingleton, IOperationSingletonInstance
{
    public Operation() : this(Guid.NewGuid())
    {
    }
    public Operation(Guid id)
    {
        OperationId = id;
    }
    public Guid OperationId { get; private set; }
}

再註冊一個OperationService服務實例，當通過依賴關係註入請求 OperationService 實例時，它將接收每個服務的新實例或基於從屬服務（Operation）的生命周期的現有實例。OperationService 服務作用就是第二次調用 Operation類，查看Operation類實例的作用域變化。

public class OperationService
{
    public OperationService(
        IOperationTransient transientOperation,
        IOperationScoped scopedOperation,
        IOperationSingleton singletonOperation,
        IOperationSingletonInstance instanceOperation)
    {
        _transientOperation = transientOperation;
        _scopedOperation = scopedOperation;
        _singletonOperation = singletonOperation;
        _singletonInstanceOperation = instanceOperation;
    }
    public IOperationTransient _transientOperation { get; }
    public IOperationScoped _scopedOperation { get; }
    public IOperationSingleton _singletonOperation { get; }
    public IOperationSingletonInstance _singletonInstanceOperation { get; }
}

然後在Startup.ConfigureServices()服務容器中註冊各個生命周期的實例：

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<IOperationTransient, Operation>();
    services.AddScoped<IOperationScoped, Operation>();
    services.AddSingleton<IOperationSingleton, Operation>();
    services.AddSingleton<IOperationSingletonInstance>(new Operation(Guid.Empty));
    // OperationService depends on each of the other Operation types.
    services.AddTransient<OperationService, OperationService>();
}

再在IndexModel模塊裡面調用OnGet方法輸出，觀察IOperation與OperationService類屬性OperationId 值的變化：

public class IndexModel : PageModel
{
    public OperationService _operationService { get; }
    public IOperationTransient _transientOperation { get; }
    public IOperationScoped _scopedOperation { get; }
    public IOperationSingleton _singletonOperation { get; }
    public IOperationSingletonInstance _singletonInstanceOperation { get; }
    public IndexModel(
    OperationService operationService,
    IOperationTransient transientOperation,
    IOperationScoped scopedOperation,
    IOperationSingleton singletonOperation,
    IOperationSingletonInstance singletonInstanceOperation)
    {
        _operationService = operationService;
        _transientOperation = transientOperation;
        _scopedOperation = scopedOperation;
        _singletonOperation = singletonOperation;
        _singletonInstanceOperation = singletonInstanceOperation;
    }
    public void OnGet()
    {
        Console.WriteLine("IOperation操作:");
        Console.WriteLine("暫時:" + _transientOperation.OperationId.ToString());
        Console.WriteLine("作用域:" + _scopedOperation.OperationId.ToString());
        Console.WriteLine("單例:" + _singletonOperation.OperationId.ToString());
        Console.WriteLine("實例:" + _singletonInstanceOperation.OperationId.ToString());
        Console.WriteLine("OperationService操作:");
        Console.WriteLine("暫時:" + _operationService._transientOperation.OperationId.ToString());
        Console.WriteLine("作用域:" + _operationService._scopedOperation.OperationId.ToString());
        Console.WriteLine("單例:" + _operationService._singletonOperation.OperationId.ToString());
        Console.WriteLine("實例:" + _operationService._singletonInstanceOperation.OperationId.ToString());
    }
}

執行IndexModel 類輸出結果：

由圖總結如下：
2.1 Transient（暫時）：每次調用服務的時候都會創建一個新的實例。即在IndexModel類的局部方法或屬性中（這裡是OnGet方法）實例化一個依賴對象Operation類，偽代碼是：

public class IndexModel: PageModel
{
    public void OnGet()
    {
          //調用IndexModel類時，實例化了兩次Operation類
　　　　  //第一次
          OperationService operationService=new OperationService();
　　　　　//第二次
　　　　　IOperationTransient TransientOperation=new Operation();
    }
}

2.2 Scoped（作用域）：一次請求（Action）內對象實例是相同的，但每次請求會產生一個新實例。相當於在IndexModel類的全局中實例化一次依賴對象Operation類，偽代碼是：

OperationService operationService = null;
public IndexModel()
{
    operationService = new OperationService();
    operationService._scopedOperation = new Operation();
}
public void OnGet()
{
    operationService._scopedOperation.OperationId;
    IOperationScoped operationScoped = operationService._scopedOperation;
    operationScoped.OperationId
}

2.3 Singleton（單例）：首次請求初始化同一個實例，後續每次請求都使用同一個實例。相當於在整個應用Application中只實例化一次實例，常見的單例模式。

參考文獻：
在ASP.NET Core依賴註入