ASP.NET Core中的依赖注入可以说是无处不在，其通过创建一个ServiceCollection对象并将服务注册信息以ServiceDescriptor对象的形式添加在其中，其次针对ServiceCollection对象创建对应的ServiceProvider，通过ServiceProvider提供我们需要的服务实例。

这里通过IServiceCollection来查看一下 其默认注册了哪些服务。

• 1、使用asp.net core mvc默认模板，命令行创建项目

dotnet new mvc -o projectname

Microsoft.Extensions.DependencyInjection中提供很多IServiceCollection的扩展方法来添加服务注册。如：

/// <summary>
/// Adds MVC services to the specified <see cref="IServiceCollection" />.
/// </summary>
/// <param name="services">The <see cref="IServiceCollection" /> to add services to.</param>
/// <returns>An <see cref="IMvcBuilder"/> that can be used to further configure the MVC services.</returns>
public static IMvcBuilder AddMvc(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    services.AddControllersWithViews();
    return services.AddRazorPages();
}

public static IMvcBuilder AddControllers(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    var builder = AddControllersCore(services);
    return new MvcBuilder(builder.Services, builder.PartManager);
}

• 2、在Startup.cs中 将IServiceCollection中注册的服务输出到日志

private IServiceCollection _services;

public Startup(IConfiguration configuration)
{
    Configuration = configuration;
}

public IConfiguration Configuration { get; }

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });


    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

    _services = services;

}
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    var _logger = loggerFactory.CreateLogger("Services");
    _logger.LogInformation($"Total Services Registered: {_services.Count}");
    foreach (var service in _services)
    {
        _logger.LogInformation($"Service: {service.ServiceType.FullName}\n      Lifetime: {service.Lifetime}\n      Instance: {service.ImplementationType?.FullName}");
    }
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
        // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();
    app.UseCookiePolicy();

    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });
}

• 3、我们能够直接获取这些服务的实例来使用，也可以使用第三方容器如：AspectCore、Autofac

例：使用AspectCore

public class AspectCoreContainer
{
    private static IServiceResolver resolver { get; set; }
    public static IServiceProvider BuildServiceProvider(IServiceCollection services, Action<IAspectConfiguration> configure = null)
    {
        if(services==null)throw new ArgumentNullException(nameof(services));
        services.ConfigureDynamicProxy(configure);
        services.AddAspectCoreContainer();
        return resolver = services.ToServiceContainer().Build();
    }

    public static T Resolve<T>()
    {
        if (resolver == null)
            throw new ArgumentNullException(nameof(resolver), "调用此方法时必须先调用BuildServiceProvider！");
        return resolver.Resolve<T>();
    }
    public static List<T> ResolveServices<T>()
    {
        if (resolver == null)
            throw new ArgumentNullException(nameof(resolver), "调用此方法时必须先调用BuildServiceProvider！");
        return resolver.GetServices<T>().ToList();
    }
}

调用Resolve拿到实例

var razorViewEngine = AspectCoreContainer.Resolve<IRazorViewEngine>();
var compositeViewEngine = AspectCoreContainer.Resolve<ICompositeViewEngine>();
var tempDataProvider = AspectCoreContainer.Resolve<ITempDataProvider>();
var serviceProvider = AspectCoreContainer.Resolve<IServiceProvider>();
var options = AspectCoreContainer.Resolve<IOptions>();