到目前为止，我们定义的ServiceProvider已经实现了基本的服务提供和回收功能，但是依然漏掉了一些必需的细节特性。这些特性包括如何针对IServiceProvider接口提供一个ServiceProvider对象，何创建ServiceScope，以及如何提供一个服务实例的集合。
一、提供一个ServiceProvider对象
我们知道当将服务类型指定为IServiceProvider接口并调用ServiceProvider的GetService方法是，ServiceProvider对象本身将会作为服务实例返回，这个特性可以利用一个自定义的Service来实现。如下面的代码片段所示，我们定义的这个ServiceProviderService既是一个Service，又是一个ServiceCallSite。它默认采用生命周期管理模式为Scoped，在Invoke和Build方法中，它直接将当前ServiceProvider作为提供的服务实例。在初始化ServiceTable的时候，我们额外添加一个针对ServiceProviderService的ServideEntry。
代码语言：javascript复制 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并添加相应ServiceEntry
27: this.ServieEntries[typeof(IServiceProvider)] = new ServiceEntry(new ServiceProviderService());
28: }
29: }二、创建ServiceScope
创建ServiceScope的目的在于创建作为当前ServiceProvider儿子的另一个ServiceProvider，新创建的ServiceProvider不仅与原来的ServiceProvider具有相同的根，同时共享所有的服务注册信息。利用这个新的ServiceProvider来代替现有的ServiceProvider，其主要的目的还是使我们能够及时地回收提供的服务实例。ServiceScope是通过它的工厂ServiceScopeFactory来创建的，所以先创建了如下一个ServiceScopeFactory类和对应的ServiceScope，它们的定义与我们在前面一节介绍的完全一致。
代码语言：javascript复制 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: }为了让ServiceProvider的GetService方法在服务类型指定为IServiceScopeFactory接口的时候能够自动返回上面我们定义的ServiceScopeFactory对象，我们依然和上面一样创建了一个自定义的Service，并将其命名为ServiceScopeFactoryService。与ServiceProviderService一样，ServiceScopeFactoryService同时也是一个ServiceCallSite，在Build和Invoke方法中它会返回一个ServiceScopeFactory对象。为了让这个它能够生效，我们依然在ServiceTable初始化的时自动添加一个相应的ServiceEntry。
代码语言：javascript复制 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并添加相应ServiceEntry
27: this.ServieEntries[typeof(IServiceProvider)] = new ServiceEntry(new ServiceProviderService());
28: this.ServieEntries[typeof(IServiceScopeFactory)] = new ServiceEntry(new ServiceScopeFactoryService());
29: }
30: }三、提供一组服务的集合
到目前为止，我们自定义的ServiceProvider尚不具备原生ServiceProvider的一项特性，那就是当调用GetService方法时将服务类型指定为IEnumerable<T>或者直接调用扩展方法GetServices时，得到的是一个服务实例的集合。这个特性可以通过一个自定义的ServiceCallSite来完成，我们将其命名为EnumerableCallSite。
代码语言：javascript复制 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: }
17:
18: public object Invoke(ServiceProvider provider)
19: {
20: var array = Array.CreateInstance(this.ElementType, this.ServiceCallSites.Length);
21: for (var index = 0; index < this.ServiceCallSites.Length; index )
22: {
23: array.SetValue(this.ServiceCallSites[index].Invoke(provider), index);
24: }
25: return array;
26: }
27: }如上面的代码片段所示，EnumerableCallSite具有两个两个只读属性（ElementType和ServiceCallSites），前者表示返回的服务集合的元素类型，后者则返回一组用于提供集合元素的ServiceCallSite。在Invoke和Build方法中，我们只需要根据元素类型创建一个数组，并利用这组ServiceCallSite创建所有的元素即可。这个EnumerableCallSite最终按照如下的方式应用到ServiceProvider的GetServiceCallSite方法中。
代码语言：javascript复制 1: internal class ServiceProvider : IServiceProvider, IDisposable
2: {
3: public IServiceCallSite GetServiceCallSite(Type serviceType, ISet<Type> callSiteChain)
4: {
5: try
6: {
7: if (callSiteChain.Contains(serviceType))
8: {
9: throw new InvalidOperationException(string.Format("A circular dependency was detected for the service of type '{0}'",serviceType.FullName);
10: }
11: callSiteChain.Add(serviceType);
12: ServiceEntry serviceEntry;
13: if (this.ServiceTable.ServieEntries.TryGetValue(serviceType, out serviceEntry))
14: {
15: return serviceEntry.Last.CreateCallSite(this, callSiteChain);
16: }
17:
18: if (serviceType.IsGenericType && serviceType.GetGenericTypeDefinition()== typeof(IEnumerable<>))
19: {
20: Type elementType = serviceType.GetGenericArguments()[0];
21: IServiceCallSite[] serviceCallSites = this.ServiceTable.ServieEntries.TryGetValue(elementType, out serviceEntry)
22: ? serviceEntry.All.Select(it => it.CreateCallSite(this, callSiteChain)).ToArray()
23: : new IServiceCallSite[0];
24: return new EnumerableCallSite(elementType, serviceCallSites);
25: }
26:
27: return null;
28: }
29: finally
30: {
31: callSiteChain.Remove(serviceType);
32: }
33: }
34: //其他成员
35: }
