接着 上篇 目前也算是交代清楚了相关的类。那么框架具体是如何来实例化的呢？整个的流程是怎么样的。
我们参考源码中的Test文件夹来看看：
var collection = new ServiceCollection();collection.AddTransient<DependOnNonexistentService>();var provider = CreateServiceProvider(collection);protected override IServiceProvider CreateServiceProvider(IServiceCollection collection) => collection.BuildServiceProvider();public static ServiceProvider BuildServiceProvider(this IServiceCollection services){ ???return BuildServiceProvider(services, ServiceProviderOptions.Default);}public class ServiceProviderOptions{ ???// Avoid allocating objects in the default case ???internal static readonly ServiceProviderOptions Default = new ServiceProviderOptions(); ???/// <summary> ???/// <c>true</c> to perform check verifying that scoped services never gets resolved from root provider; otherwise <c>false</c>. ???/// </summary> ???public bool ValidateScopes { get; set; } ???internal ServiceProviderMode Mode { get; set; } = ServiceProviderMode.Dynamic;}internal enum ServiceProviderMode{ ???Dynamic, ???Runtime, ???Expressions, ???ILEmit}public static ServiceProvider BuildServiceProvider(this IServiceCollection services, ServiceProviderOptions options){ ???if (services == null) ???{ ???????throw new ArgumentNullException(nameof(services)); ???} ???if (options == null) ???{ ???????throw new ArgumentNullException(nameof(options)); ???} ???return new ServiceProvider(services, options);}internal ServiceProvider(IEnumerable<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options){ ???IServiceProviderEngineCallback callback = null; ???if (options.ValidateScopes) ???{ ???????callback = this; ???????_callSiteValidator = new CallSiteValidator(); ???} ???switch (options.Mode) ???{ ???????case ServiceProviderMode.Dynamic: ???????????_engine = new DynamicServiceProviderEngine(serviceDescriptors, callback); ???????????break; ???????case ServiceProviderMode.Runtime: ???????????_engine = new RuntimeServiceProviderEngine(serviceDescriptors, callback); ???????????break;#if IL_EMIT ???????case ServiceProviderMode.ILEmit: ???????????_engine = new ILEmitServiceProviderEngine(serviceDescriptors, callback); ???????????break;#endif ???????case ServiceProviderMode.Expressions: ???????????_engine = new ExpressionsServiceProviderEngine(serviceDescriptors, callback); ???????????break; ???????default: ???????????throw new NotSupportedException(nameof(options.Mode)); ???}}如上所示，就是注入服务，实例化服务的整个过程了。我们主要是根据IServiceCollection的扩展方法BuilServiceProvider调用ServiceProvider类，进而实例化相关服务。
除去这种方式实现ServiceProvider外，默认asp.net core框架中还有一个DefaultServiceProviderFactory。不过这个工厂类最后也是调用IServiceCollection接口的拓展方法--BuildServiceProvider来创建ServiceProvider类。
总结：
IServiceCollection接口的拓展方法Addxxx 收集需要注入的服务，服务实现类型，服务的生命周期。这些就是在ServiceDescriptor中获取到的，也是该类的属性对象。这个也是IServiceCollection继承自IList<ServiceDescriptor>的解释所在。随后再获取ServiceProviderOptions枚举对象，选择实现实例化的类型，比如反射等。最后就是调用BuildServiceProvider来生成ServiceProvider了。ServiceProvider根据选择之前的选择及配置，实例化服务。
asp.net core 依赖注入实现全过程粗略剖析（2）
原文地址：https://www.cnblogs.com/zhiyong-ITNote/p/9388277.html