Dependency Injection in ASP.net Session_Start method

Question

I am learning dependency injection and using autofac for the first time. I built the container as mentioned in several autofac examples (see below) and called from my application_start

public class ContainerConfig
{
    public static void RegisterContainer() 
    {
        //Create a new ContainerBuilder
        var builder = new ContainerBuilder();

        // Register all the controllers using the assembly object
        builder.RegisterControllers(Assembly.GetExecutingAssembly());

        //Registering default convention -- IExample and Example
        builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
            .Where(t => t.Name.Single(i => i.Name == "I" + t.Name))
            .AsImplementedInterfaces();

        //Build the container
        var container = builder.Build();

        //Set the default resolver to use Autofac
        DependencyResolver.SetResolver(new AutofacDependencyResolver(container)); 
    }
}

I created UserService in my Core project with IUserService. This has methods to make dbcall to get user information from tables. In my UI project, i have a class called UserProvider to which i am tying to inject UserService.

public class UserProvider
{
    private readonly IUserService _userService;

    public UserProvider(IUserService userService) 
    {
        _userService = userService;
    }

    public void LoadCurrentUser()
    {
        Users FoundUser = _userService.ImportOrGetUser();

        if (FoundUser != null)
        {
            //add it to session
            CurrentUser = FoundUser;
        }
    }
}

This UserProvider, i am using in my session_start

void Session_OnStart()
{
     UserProvider OUsrPrv = new UserProvider(new UserService());
     OUsrPrv.LoadCurrentUser();
}

In the above code, if i am passing 'new UserService()', my understanding is i am injecting UserService manually. I dont see how autofac is helping here. All the examples in google are talking about Dependency injection in MVCController or WebApiController, not in a individual class (UserProvider) like i am doing.

Can somebody please throw some light? Am I doing it all wrong?

Answer 1

In order to properly use Dependency Injection, you should never create instance by yourself, the underlying framework should provide instances for you.

But ASP.net invokes the Session_OnStart without any Dependency Injection. In this case you can use the DependencyResolver.Current static property to resolve the requested service.

void Session_OnStart()
{
    UserProvider userProvider = DependencyResolver.Current.GetService<UserProvider>();
    userProvider.LoadCurrentUser(); 
}

Answer 2

The event model in the System.Web.HttpApplication is part of ASP.NET, not MVC. It was not designed for use with dependency injection.

The answer that Cyril suggested is using a service locator to get a reference to the service. This is far from ideal, since you are taking on a dependency to the service locator in your code.

The MVC-centric way of implementing cross cutting concerns (such as loading user data into session state) is to use globally registered filters. You can either implement IAuthorizationFilter or IActionFilter to get the desired effect. In this case it makes sense to use IActionFilter since you want to wait until you are sure there is an authorized user before it is called.

NOTE: While this answers your specific question, it is best not to use session state for this scenario in MVC. An alternative is to use ASP.NET Identity with Claims to store user profile data instead of using Session.

using System;
using System.Web.Mvc;
using System.Security.Principal;

public class GetUserActionFilter : IActionFilter
{
    private readonly IUserRepository userRepository;

    public GetUserActionFilter(IUserRepository userRepository)
    {
        if (userRepository == null)
            throw new ArgumentNullException("userRepository");

        this.userRepository = userRepository;
    }

    public void OnActionExecuted(ActionExecutedContext filterContext)
    {
        // Do nothing - this occurs after the action method has run
    }

    public void OnActionExecuting(ActionExecutingContext filterContext)
    {
        IPrincipal user = filterContext.HttpContext.User;

        if (user == null)
        {
            return;
        }

        IIdentity identity = user.Identity;

        if (identity == null)
        {
            return;
        }

        // Make sure we have a valid identity and it is logged in.
        if (identity.IsAuthenticated)
        {
            string key = "__CurrentUserData";
            var userData = filterContext.HttpContext.Session[key];
            if (userData == null)
            {
                // User data doesn't exist in session, so load it
                userData = userRepository.GetUserData(identity.Name);

                // Add it to session state
                filterContext.HttpContext.Session[key] = userData;
            }
        }
    }
}

Now, to add your filter globally, you need to:

1. Register the filter and its dependencies with Autofac.
2. Pass the container to the static RegisterGlobalFilters method.

Register the Filter

Using a named instance to differentiate it from other potential IActionFilter instances.

builder.RegisterType<GetUserActionFilter>()
       .Named<IActionFilter>("getUserActionFilter");

Pass the Container

FilterConfig.cs

public class FilterConfig
{
    public static void RegisterGlobalFilters(GlobalFilterCollection filters, IContainer container)
    {
        filters.Add(container.ResolveNamed<IActionFilter>("getUserActionFilter"));
        filters.Add(new HandleErrorAttribute());
    }
}

Global.asax.cs

public class MvcApplication : System.Web.HttpApplication
{
    // This method serves as the composition root
    // for the project.
    protected void Application_Start()
    {
        // Register Autofac DI
        IContainer container = ContainerConfig.RegisterContainer();

        AreaRegistration.RegisterAllAreas();

        WebApiConfig.Register(GlobalConfiguration.Configuration);
        FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters, container);
        RouteConfig.RegisterRoutes(RouteTable.Routes);
        BundleConfig.RegisterBundles(BundleTable.Bundles);
        AuthConfig.RegisterAuth();
    }
}

ContainerConfig.cs

public class ContainerConfig
{
    public static IContainer RegisterContainer() 
    {
        //Create a new ContainerBuilder
        var builder = new ContainerBuilder();

        // Register all the controllers using the assembly object
        builder.RegisterControllers(Assembly.GetExecutingAssembly());

        //Registering default convention -- IExample and Example
        builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
              .Where(t => t.Name.Single(i => i.Name == "I" + t.Name))
              .AsImplementedInterfaces();

        // Register our filter
        builder.RegisterType<GetUserActionFilter>()
           .Named<IActionFilter>("getUserActionFilter");

        //Build the container
        var container = builder.Build();

        //Set the default resolver to use Autofac
        DependencyResolver.SetResolver(new AutofacDependencyResolver(container)); 

        // Return the container to our composition root.
        return container;
    }
}

Note that I just used a repository service here, since HttpContext is available already through the action filter directly and additional logic is needed here because we don't know for sure if it exists in session state or not or whether there is even a user to lookup, so our filter does those checks in addition to loading session state.