转载请注明出处： https://home.cnblogs.com/u/zhiyong-ITNote/
常用扩展方法 注入依赖服务：
代码语言：javascript复制new ServiceCollection().AddSingleton<IApplicationBuilder, ApplicationBuilder>();// AddSingleton多个重载方法 源码
代码语言：javascript复制public static IServiceCollection AddSingleton<TService, TImplementation>(this IServiceCollection services)
where TService : class
where TImplementation : class, TService
{
if (services == null)
{
throw new ArgumentNullException(nameof(services));
}
return services.AddSingleton(typeof(TService), typeof(TImplementation));
}
// Singleton模式 最终的调用
public static IServiceCollection AddSingleton(
this IServiceCollection services,
Type serviceType,
Type implementationType)
{
if (services == null)
{
throw new ArgumentNullException(nameof(services));
}
if (serviceType == null)
{
throw new ArgumentNullException(nameof(serviceType));
}
if (implementationType == null)
{
throw new ArgumentNullException(nameof(implementationType));
}
return Add(services, serviceType, implementationType, ServiceLifetime.Singleton);
}
// 所有的Addxxx 最终都是调用Add方法，将ServiceDescriptor添加到IServiceCollection中：
private static IServiceCollection Add(
IServiceCollection collection,
Type serviceType,
Type implementationType,
ServiceLifetime lifetime)
{
var descriptor = new ServiceDescriptor(serviceType, implementationType, lifetime);
collection.Add(descriptor);
return collection;
}
// IServiceCollection源码：
public interface IServiceCollection : IList<ServiceDescriptor>
{}如上，我们一般在ConfigureService中使用Addxxx将服务注入框架的过程。大概做个总结，其实就是屌用IServiceCollection的Addxxx 扩展方法，随后调用Add方法，初始化一个ServiceDescriptor，参数是我们注入的接口和类，还有就是生命周期。随后添加到IServiceCollection中，根据该接口的定义就是一个ServiceDescriptor的集合。
我们看看ServiceDescriptor的源码：
代码语言：javascript复制public ServiceDescriptor(
Type serviceType,
object instance)
: this(serviceType, ServiceLifetime.Singleton)
{
if (serviceType == null)
{
throw new ArgumentNullException(nameof(serviceType));
}
if (instance == null)
{
throw new ArgumentNullException(nameof(instance));
}
ImplementationInstance = instance;
}该类就是初始化并获取如下属性：
代码语言：javascript复制/// <inheritdoc />
public ServiceLifetime Lifetime { get; }
/// <inheritdoc />
public Type ServiceType { get; }
/// <inheritdoc />
public Type ImplementationType { get; }
/// <inheritdoc />
public object ImplementationInstance { get; }
/// <inheritdoc />
public Func<IServiceProvider, object> ImplementationFactory { get; }到此，我们的服务注入到asp.net core框架中就完事了，那么服务的实例化呢？
首先我们理下思路，IServiceProvider接口对应的实现是ServiceProvider,这个类就是实例化了IServiceProvider接口，而IServiceProvider接口只有一个方法：
代码语言：javascript复制public object GetService(Type serviceType);该方法就是获取注入的服务。但是ServiceProvider类不单单是获取注入的服务，服务的实例化还是在该类中实现的，我们看下：
代码语言：javascript复制public sealed class ServiceProvider : IServiceProvider, IDisposable, IServiceProviderEngineCallback
{
private readonly IServiceProviderEngine _engine;
private readonly CallSiteValidator _callSiteValidator;
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));
}
}
/// <summary>
/// Gets the service object of the specified type.
/// </summary>
/// <param name="serviceType"></param>
/// <returns></returns>
public object GetService(Type serviceType) => _engine.GetService(serviceType);
/// <inheritdoc />
public void Dispose() => _engine.Dispose();
void IServiceProviderEngineCallback.OnCreate(IServiceCallSite callSite)
{
_callSiteValidator.ValidateCallSite(callSite);
}
void IServiceProviderEngineCallback.OnResolve(Type serviceType, IServiceScope scope)
{
_callSiteValidator.ValidateResolution(serviceType, scope, _engine.RootScope);
}
}可以看到该类的构造函数中就是实例化服务的过程了。很直白的可以看出我们常见的几种方法来实例化类：反射，Emit，表达式树等...
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。篇2再叙
源码地址：https://github.com/aspnet/DependencyInjection
