（3）ASP.NET Core 服务生命周期

1.前言

在ConfigureServices方法中的容器注册每个应用程序的服务，Asp.Core都可以为每个应用程序提供三种服务生命周期：
●Transient（暂时）：每次请求都会创建一个新的实例。这种生命周期最适合轻量级，无状态服务。
●Scoped（作用域）：在同一个作用域内只初始化一个实例，可以理解为每一个请求只创建一个实例，同一个请求会在一个作用域内。
●Singleton（单例）：整个应用程序生命周期以内只创建一个实例，后续每个请求都使用相同的实例。如果应用程序需要单例行为，建议让服务容器管理服务的生命周期，而不是在自己的类中实现单例模式。

2.服务生命周期与注册选项案例演示

为了演示生命周期和注册选项之间的差异，请考虑以下接口，将任务表示为具有唯一标识符 OperationId 的操作。根据以下接口配置操作服务的生命周期的方式，容器在类请求时提供相同或不同的服务实例：

public interface IOperation

{

    Guid OperationId { get; }

}

public interface IOperationTransient : IOperation

{

}

public interface IOperationScoped : IOperation

{

}

public interface IOperationSingleton : IOperation

{

}

public interface IOperationSingletonInstance : IOperation

{

}

上面四种服务接口在 Operation 类中实现。调用Operation类时将自动生成一个GUID，下面是Operation类的实现：

public class Operation : IOperationTransient, IOperationScoped, IOperationSingleton, IOperationSingletonInstance

{

    public Operation() : this(Guid.NewGuid())

    {

    }

    public Operation(Guid id)

    {

        OperationId = id;

    }

    public Guid OperationId { get; private set; }

}

再注册一个OperationService服务实例，当通过依赖关系注入请求 OperationService 实例时，它将接收每个服务的新实例或基于从属服务（Operation）的生命周期的现有实例。OperationService 服务作用就是第二次调用 Operation类，查看Operation类实例的作用域变化。

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; }

}

然后在Startup.ConfigureServices()服务容器中注册各个生命周期的实例：

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模块里面调用OnGet方法输出，观察IOperation与OperationService类属性OperationId 值的变化：

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("暂时:" + _transientOperation.OperationId.ToString());

        Console.WriteLine("作用域:" + _scopedOperation.OperationId.ToString());

        Console.WriteLine("单例:" + _singletonOperation.OperationId.ToString());

        Console.WriteLine("实例:" + _singletonInstanceOperation.OperationId.ToString());

        Console.WriteLine("OperationService操作:");

        Console.WriteLine("暂时:" + _operationService._transientOperation.OperationId.ToString());

        Console.WriteLine("作用域:" + _operationService._scopedOperation.OperationId.ToString());

        Console.WriteLine("单例:" + _operationService._singletonOperation.OperationId.ToString());

        Console.WriteLine("实例:" + _operationService._singletonInstanceOperation.OperationId.ToString());

    }

}

执行IndexModel 类输出结果：

由图总结如下：
2.1 Transient（暂时）：每次调用服务的时候都会创建一个新的实例。即在IndexModel类的局部方法或属性中（这里是OnGet方法）实例化一个依赖对象Operation类，伪代码是：

public class IndexModel: PageModel

{

    public void OnGet()

    {

          //调用IndexModel类时，实例化了两次Operation类

　　　　  //第一次

          OperationService operationService=new OperationService();

　　　　　//第二次

　　　　　IOperationTransient TransientOperation=new Operation();

    }

}

2.2 Scoped（作用域）：一次请求（Action）内对象实例是相同的，但每次请求会产生一个新实例。相当于在IndexModel类的全局中实例化一次依赖对象Operation类，伪代码是：

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（单例）：首次请求初始化同一个实例，后续每次请求都使用同一个实例。相当于在整个应用Application中只实例化一次实例，常见的单例模式。

参考文献：
在ASP.NET Core依赖注入

巴特西

（3）ASP.NET Core 服务生命周期

1.前言

2.服务生命周期与注册选项案例演示

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