autofac解析Mvc和Webapi的坑

我们在项目中很早就开始使用autofac，也以为知道与mvc和webapi集成的做法。

var builder = new ContainerBuilder();

// Mvc Register
builder.RegisterControllers(Assembly.GetExecutingAssembly()).AsSelf().PropertiesAutowired();
builder.RegisterFilterProvider();
builder.RegisterType<UserService>().As<IUserService>().InstancePerLifetimeScope();

//WebApi Register
builder.RegisterApiControllers(Assembly.GetExecutingAssembly()).AsSelf().PropertiesAutowired();
builder.RegisterWebApiFilterProvider(GlobalConfiguration.Configuration);
builder.RegisterWebApiModelBinderProvider();

var container = builder.Build();

// Set the dependency resolver for Web API.
var webApiResolver = new AutofacWebApiDependencyResolver(container);
GlobalConfiguration.Configuration.DependencyResolver = webApiResolver;

// Set the dependency resolver for MVC.
var resolver = new AutofacDependencyResolver(container);
DependencyResolver.SetResolver(resolver);

在实际Controller和ApiController中通过构造函数注入，这不必多说。

但是，在实际项目需求的时候，有些地方不方便使用构造函数，或者说就要使用服务定位IContainer.Resolve(ServiceType)的方式来获得服务的实例。

曾经在项目中看到过有人通过把Container设区全局静态变量来获得对象容器。这个方式在Local的情况下，不会有太大问题。在Mvc中，容器DependencyResolver.Current本身也是通过尽量变量来实现的。

    public class DependencyResolver
    {
        public DependencyResolver();

        public static IDependencyResolver Current { get; }
    ...
    }

但是和C端不同的是，Web服务是基于请求的，autofac内部的InstancePerLifetimeScope，InstancePerHttpRequest，InstancePerApiRequest等都是基于每次请求的Scope,而静态的Container明显生命周期不符合。

所以我们写代码的时候都是通过DependencyResolver.Current.GetService()和GlobalConfiguration.Configuration.DependencyResolver.GetService()来分别获取Mvc和WebApi的对象。那么问题来了，我有一段业务逻辑在BLL中，Mvc和WebApi可以都调用到，其中需要Resolve一个服务，那么如何来指定容器呢？

带着问题，我们先来看看DependencyResolver.Current和GlobalConfiguration.Configuration.DependencyResolver，通过一组实验来对比一下：

    public class WebApiApplication : System.Web.HttpApplication
    {
        public static System.Web.Mvc.IDependencyResolver mvcResolver;
        public static System.Web.Http.Dependencies.IDependencyResolver apiResolver;

        protected void Application_Start()
        {
            ...
　　　　　　　builder.RegisterType<UserService>().As<IUserService>().InstancePerLifetimeScope();

             // Set the dependency resolver for Web API.
            var webApiResolver = new AutofacWebApiDependencyResolver(container);
            apiResolver = webApiResolver;
            GlobalConfiguration.Configuration.DependencyResolver = webApiResolver;

            // Set the dependency resolver for MVC.
            var resolver = new AutofacDependencyResolver(container);
            mvcResolver = resolver;
            DependencyResolver.SetResolver(mvcResolver);
          }
    }
            
　　 public interface IUserService
    {
    }

    public class UserService : IUserService
    {
    }

我们分别定义了两个静态变量mvcResolver和apiResolver来存储两个不同的容器，并注册了一组服务，指定其生命周期为InstancePerLifetimeScope，先看看Mvc的容器

    public class HomeController : Controller
    {
        IUserService _userservice;

        public HomeController(IUserService userService)
        {
            _userservice = userService;
            var a = DependencyResolver.Current.GetService<IUserService>();
            var b = WebApiApplication.mvcResolver.GetService<IUserService>();
            var c1 = ReferenceEquals(userService, a);   //true
            var c2 = ReferenceEquals(userService, b);   //true
            var c3 = ReferenceEquals(b, a);             //true
        }
    }

我们在HomeContorller中通过三种不同方法来获取对象，目的是三个对象都应该是同一个对象，结果也符合预期，再看看WebApi的:

    public class ValuesController : ApiController
    {
        IUserService _userservice;

        public ValuesController(IUserService userService)
        {
            _userservice = userService;
            var a = GlobalConfiguration.Configuration.DependencyResolver.GetService(typeof(IUserService));
            var b = WebApiApplication.apiResolver.GetService(typeof(IUserService));
            var c1 = ReferenceEquals(userService, a);   //false
            var c2 = ReferenceEquals(userService, b);   //false
            var c3 = ReferenceEquals(b, a);             //true
        }
    }

发现通过GlobalConfiguration.Configuration.DependencyResolver来获取的对象，竟然不等于构造函数解析出来的对象，有点毁三观。说明它并不是当前上下文的对象，也就是说这个对象的生命周期不在控制范围内。

那么Mvc和WebApi可不可以用同一个容器来指定呢？

我们先来看看stackoverflow上的这篇文章：Is it possible to configure Autofac to work with ASP.NET MVC and ASP.NET Web Api

其实Mvc和WebApi分别是两个独立的依赖解析器，这点没什么问题，一个是System.Web.Mvc.IDependencyResolver另一个是System.Web.Http.Dependencies.IDependencyResolver，两个互相不串。

最后，一个很重要的对象来了，那就是Autofac.IComponentContext，它就是解析的上下文，通过它来解析的对象是符合当前上下文的，我们再来看看之前的例子:

    public class HomeController : Controller
    {
        IUserService _userservice;

        public HomeController(IUserService userService, IComponentContext com)
        {
            _userservice = userService;
            var a = DependencyResolver.Current.GetService<IUserService>();
            var b = WebApiApplication.mvcResolver.GetService<IUserService>();
            var d = com.Resolve<IUserService>();
            var d1 = ReferenceEquals(userService, d);   //true
            var c1 = ReferenceEquals(userService, a);   //true
            var c2 = ReferenceEquals(userService, b);   //true
            var c3 = ReferenceEquals(b, a);             //true
        }
    }

WebApi：

    public class ValuesController : ApiController
    {
        IUserService _userservice;

        public ValuesController(IUserService userService, IComponentContext  com)
        {
            _userservice = userService;
            var a = GlobalConfiguration.Configuration.DependencyResolver.GetService(typeof(IUserService));
            var b = WebApiApplication.apiResolver.GetService(typeof(IUserService));
            var d = com.Resolve<IUserService>();
            var d1 = ReferenceEquals(userService, d);   //true
            var c1 = ReferenceEquals(userService, a);   //false
            var c2 = ReferenceEquals(userService, b);   //false
            var c3 = ReferenceEquals(b, a);             //true
        }
    }