到目前為止，我們定義的ServiceProvider已經實現了基本的服務提供和回收功能，但是依然漏掉了一些必需的細節特性。這些特性包括如何針對IServiceProvider介面提供一個ServiceProvider對象，何創建ServiceScope，以及如何提供一個服務實例的集合。

一、提供一個ServiceProvider對象

我們知道當將服務類型指定為IServiceProvider介面並調用ServiceProvider的GetService方法是，ServiceProvider對象本身將會作為服務實例返回，這個特性可以利用一個自定義的Service來實現。如下麵的代碼片段所示，我們定義的這個ServiceProviderService既是一個Service，又是一個ServiceCallSite。它預設採用生命周期管理模式為Scoped，在Invoke和Build方法中，它直接將當前ServiceProvider作為提供的服務實例。在初始化ServiceTable的時候，我們額外添加一個針對ServiceProviderService的ServideEntry。

   1: internal class ServiceProviderService : IService, IServiceCallSite

   2: {

   3:     public ServiceLifetime Lifetime => ServiceLifetime.Scoped;

   4:     public IService Next { get; set; }

   5:  

   6:     public Expression Build(Expression provider)

   7:     {

   8:         return provider;

   9:     }

  10:  

  11:     public IServiceCallSite CreateCallSite(ServiceProvider provider, ISet<Type> callSiteChain)

  12:     {

  13:         return this;

  14:     }

  15:  

  16:     public object Invoke(ServiceProvider provider)

  17:     {

  18:         return provider;

  19:     }

  20: }

  21:  

  22: internal class ServiceTable

  23: {

  24:     public ServiceTable(IServiceCollection services)

  25:     {

  26:         //解析ServiceCollection並添加相應ServiceEntry

  27:         this.ServieEntries[typeof(IServiceProvider)] = new ServiceEntry(new ServiceProviderService());

  28:     }

  29: }

二、創建ServiceScope

創建ServiceScope的目的在於創建作為當前ServiceProvider兒子的另一個ServiceProvider，新創建的ServiceProvider不僅與原來的ServiceProvider具有相同的根，同時共用所有的服務註冊信息。利用這個新的ServiceProvider來代替現有的ServiceProvider，其主要的目的還是使我們能夠及時地回收提供的服務實例。ServiceScope是通過它的工廠ServiceScopeFactory來創建的，所以先創建瞭如下一個ServiceScopeFactory類和對應的ServiceScope，它們的定義與我們在前面一節介紹的完全一致。

   1: internal class ServiceScope : IServiceScope

   2: {

   3:     public IServiceProvider ServiceProvider { get; private set; }

   4:  

   5:     public ServiceScope(ServiceProvider serviceProvider)

   6:     {

   7:         this.ServiceProvider = serviceProvider;

   8:     }

   9:  

  10:     public void Dispose()

  11:     {

  12:         (this.ServiceProvider as IDisposable)?.Dispose();

  13:     }

  14: }

  15:  

  16: internal class ServiceScopeFactory : IServiceScopeFactory

  17: {

  18:     public ServiceProvider ServiceProvider { get; private set; }

  19:  

  20:     public ServiceScopeFactory(ServiceProvider serviceProvider)

  21:     {

  22:         this.ServiceProvider = serviceProvider;

  23:     }

  24:  

  25:     public IServiceScope CreateScope()

  26:     {

  27:         return new ServiceScope(this.ServiceProvider);

  28:     }

  29: }

  30:  

  31: internal class ServiceProvider : IServiceProvider, IDisposable

  32: {

  33:     

  34:     public ServiceProvider(ServiceProvider parent)

  35:     {

  36:         this.Root = parent.Root;

  37:         this.ServiceTable = parent.ServiceTable;

  38:     }

  39: }

為了讓ServiceProvider的GetService方法在服務類型指定為IServiceScopeFactory介面的時候能夠自動返回上面我們定義的ServiceScopeFactory對象，我們依然和上面一樣創建了一個自定義的Service，並將其命名為ServiceScopeFactoryService。與ServiceProviderService一樣，ServiceScopeFactoryService同時也是一個ServiceCallSite，在Build和Invoke方法中它會返回一個ServiceScopeFactory對象。為了讓這個它能夠生效，我們依然在ServiceTable初始化的時自動添加一個相應的ServiceEntry。

   1: internal class ServiceScopeFactoryService : IService, IServiceCallSite

   2: {

   3:     public ServiceLifetime Lifetime=> ServiceLifetime.Scoped;

   4:     public IService Next { get; set; }

   5:  

   6:     public IServiceCallSite CreateCallSite(ServiceProvider provider, ISet<Type> callSiteChain)

   7:     {

   8:         return this;

   9:     }

  10:  

  11:     public Expression Build(Expression provider)

  12:     {

  13:         return Expression.New(typeof(ServiceScopeFactory).GetConstructors().Single(), provider);

  14:     }

  15:  

  16:     public object Invoke(ServiceProvider provider)

  17:     {

  18:         return new ServiceScopeFactory(provider);

  19:     }

  20: }

  21:  

  22: internal class ServiceTable

  23: {

  24:     public ServiceTable(IServiceCollection services)

  25:     {

  26:         //解析ServiceCollection並添加相應ServiceEntry

  27:         this.ServieEntries[typeof(IServiceProvider)] =  new ServiceEntry(new ServiceProviderService());

  28:         this.ServieEntries[typeof(IServiceScopeFactory)] = new ServiceEntry(new ServiceScopeFactoryService());

  29:     }

  30: }

三、提供一組服務的集合

到目前為止，我們自定義的ServiceProvider尚不具備原生ServiceProvider的一項特性，那就是當調用GetService方法時將服務類型指定為IEnumerable<T>或者直接調用擴展方法GetServices時，得到的是一個服務實例的集合。這個特性可以通過一個自定義的ServiceCallSite來完成，我們將其命名為EnumerableCallSite。

   1: internal class EnumerableCallSite : IServiceCallSite

   2: {

   3:     public Type ElementType { get; private set; }

   4:     public IServiceCallSite[] ServiceCallSites { get; private set; }

   5:  

   6:     public EnumerableCallSite(Type elementType, IServiceCallSite[] serviceCallSites)

   7:     {

   8:         this.ElementType = elementType;

   9:         this.ServiceCallSites = serviceCallSites;

  10:     }

  11:  

  12:     public Expression Build(Expression provider)

  13:     {

  14:         return Expression.NewArrayInit(this.ElementType, this.ServiceCallSites.Select(

  15:             it => Expression.Convert(it.Build(provider), this.ElementType)));

  16:     }