概述

ASP.NET Core 支持依赖关系注入 (DI) 软件设计模式，这是一种在类及其依赖关系之间实现控制反转 (IoC) 的技术。

默认服务容器是 Microsoft.Extensions.DependencyInjection 。

内置的服务容器一般能满足简单的框架和部分消费者应用的需求。 建议使用内置容器，除非你需要的特定功能不受内置容器支持，例如：

属性注入

基于名称的注入

子容器

自定义生存期管理

对迟缓初始化的 Func<T> 支持

基于约定的注册

而大部分情况下，实际项目中往往是比较复杂的，所以可以使用其他第三方IOC容器，如Autofac;

Autofac

Autofac 是.Net世界中最常用的依赖注入框架之一. 相比.Net Core标准的依赖注入库, 它提供了更多高级特性, 比如动态代理和属性注入.

优点： 

它是C#语言联系很紧密，也就是说C#里的很多编程方式都可以为Autofac使用，例如可以用Lambda表达式注册组件

较低的学习曲线，学习它非常的简单，只要你理解了IoC和DI的概念以及在何时需要使用它们

XML配置支持

自动装配

具体使用

1、项目中引用

 <PackageReference Include="Autofac" Version="4.9.1" /><PackageReference Include="Autofac.Extensions.DependencyInjection" Version="4.3.1" />

2、准备好类、接口

using System.Collections.Generic;namespace AspNetCoreExample.Services
{public interface IValuesService{IEnumerable<string> FindAll();string Find(int id);}
}

using System.Collections.Generic;
using Microsoft.Extensions.Logging;namespace AspNetCoreExample.Services
{public class ValuesService : IValuesService{private readonly ILogger<ValuesService> _logger;public ValuesService(ILogger<ValuesService> logger){_logger = logger;}public IEnumerable<string> FindAll(){_logger.LogDebug("{method} called", nameof(FindAll));return new[] { "value1", "value2" };}public string Find(int id){_logger.LogDebug("{method} called with {id}", nameof(Find), id);return $"value{id}";}}
}

3、替换掉内置的Ioc

using System;
using System.Linq;
using Autofac.Extensions.DependencyInjection;
using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;namespace AspNetCoreExample
{public class Program{public static void Main(string[] args){// The ConfigureServices call here allows for// ConfigureContainer to be supported in Startup with// a strongly-typed ContainerBuilder. If you don't// have the call to AddAutofac here, you won't get// ConfigureContainer support. This also automatically// calls Populate to put services you register during// ConfigureServices into Autofac.var host = WebHost.CreateDefaultBuilder(args).ConfigureServices(services => services.AddAutofac()).UseStartup<Startup>().Build();host.Run();}}
}

using System;
using System.Linq;
using Autofac;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;namespace AspNetCoreExample
{// ASP.NET Core docs for Autofac are here:// https://autofac.readthedocs.io/en/latest/integration/aspnetcore.htmlpublic class Startup{public Startup(IHostingEnvironment env){var builder = new ConfigurationBuilder().SetBasePath(env.ContentRootPath).AddJsonFile("appsettings.json", optional: true, reloadOnChange: true).AddEnvironmentVariables();this.Configuration = builder.Build();}public IConfigurationRoot Configuration { get; private set; }public void Configure(IApplicationBuilder app){app.UseMvc();}public void ConfigureContainer(ContainerBuilder builder){// Add any Autofac modules or registrations.// This is called AFTER ConfigureServices so things you// register here OVERRIDE things registered in ConfigureServices.//// You must have the call to AddAutofac in the Program.Main// method or this won't be called.builder.RegisterModule(new AutofacModule());}public void ConfigureServices(IServiceCollection services){// Use extensions from libraries to register services in the// collection. These will be automatically added to the// Autofac container.//// Note if you have this method return an IServiceProvider// then ConfigureContainer will not be called.services.AddMvc();}}
}

4、注入我们准备好的测试类、接口

using Autofac;
using AspNetCoreExample.Services;
using Microsoft.Extensions.Logging;namespace AspNetCoreExample
{public class AutofacModule : Module{protected override void Load(ContainerBuilder builder){// The generic ILogger<TCategoryName> service was added to the ServiceCollection by ASP.NET Core.// It was then registered with Autofac using the Populate method. All of this starts// with the services.AddAutofac() that happens in Program and registers Autofac// as the service provider.builder.Register(c => new ValuesService(c.Resolve<ILogger<ValuesService>>())).As<IValuesService>().InstancePerLifetimeScope();}}
}

5、小试牛刀，用起来

using System.Collections.Generic;
using AspNetCoreExample.Services;
using Microsoft.AspNetCore.Mvc;namespace AspNetCoreExample.Controllers
{/// <summary>/// Simple REST API controller that shows Autofac injecting dependencies./// </summary>/// <seealso cref="Microsoft.AspNetCore.Mvc.Controller" />[Route("api/[controller]")]public class ValuesController : Controller{private readonly IValuesService _valuesService;public ValuesController(IValuesService valuesService){this._valuesService = valuesService;}// GET api/values[HttpGet]public IEnumerable<string> Get(){return this._valuesService.FindAll();}// GET api/values/5[HttpGet("{id}")]public string Get(int id){return this._valuesService.Find(id);}}
}