我可以在MvcApplication中输入一些内容吗？

Question

I'm using ASP.NET MVC 5, with Ninject 3.

I want to have some code run when the web site starts, which will pull something from the database. I already have injection set up, and it's working fine elsewhere in the project.

I changed the Global.asax file to look like this...

[Inject]
public VoucherCompanyServiceLogicInterface VoucherCompanyServiceLogic { get; set; }

protected void Application_Start() {
  AreaRegistration.RegisterAllAreas();
  FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
  RouteConfig.RegisterRoutes(RouteTable.Routes);
  BundleConfig.RegisterBundles(BundleTable.Bundles);
  VoucherCompany vc = VoucherCompanyServiceLogic.GetByID(1, 1);
}

...but the VoucherCompanyServiceLogic property is always null. I copied the two lines for this (the Inject attribute and the property declaration) from another class where they work fine.

I put some Debug.WriteLine statements in this method, and in the RegisterServices method of the NinjectWebCommon class, and I can see that the services are being registered before I try to use this one, so that's not the issue.

Anyone any idea why this is always null?

Answer 1

The instance of MvcApplication is created for you by ASP.Net, Ninject can do nothing to automatically inject that property.

As you mention that injection works elsewhere in your MVC application, I am assuming you are using one of the Nuget packages that includes a NinjectWebCommon with all the plumbing needed for DI in an MVC application. The NinjectWebCommon will be run before the Application_Start event as it likely registers a PreApplicationStartMethod and apart from registering your dependencies it is also configuring an IDependencyResolver for MVC behind the scenes.

So you could use the dependency resolver in your Application_Start method to get an instance of that service:

protected void Application_Start()
{        
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);

    VoucherCompanyServiceLogic = DependencyResolver.Current.GetService<IVoucherCompanyServiceLogicInterface>();
    if(VoucherCompanyServiceLogic != null)
    {
        VoucherCompany vc = VoucherCompanyServiceLogic.GetByID(1, 1);
    }
}

But what are you planning to do with that VoucherCompany instance? Are you aware that multiple instances of the MvcApplication are likely to be created by ASP.Net, and Application_Start will NOT be called for every instance? See this other SO question or read this article .

That means you could end with instances of the HttpApplication class where both the service and the company are null. If you just wanted to get the company on application startup and have that object available on subsequent events, I would set VoucherCompany as static, populate it on Application_Start and wouldn't even bother to save the reference to the service. (Depending on what you want to achieve, there might be better options.)

Hope it helps!