ASP.NET Core 依赖注入-集成 Autofac

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 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



AutoFac的使用

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.html
    public 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);
        }
    }
}