（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