HttpContextBase.Request exception when using Ninject MVC3 - asp.net-mvc-4

Question

I have a service that takes a dependency on HttpContextBase.
Ninject is injecting this for me already as it's set up in the MvcModule to return new HttpContextWrapper(HttpContext.Current) when HttpContextBase is requested
I want to use this service in Application_AuthenticateRequest, so i'm using property injection so that Ninject resolves it for me
When I try and access Request.UserHostAddress on the HttpContextBase I get a Value does not fall within the expected range exception
If I call HttpContext.Current.Request.UserHostAddress directly it works without problems
ExampleService.cs
public class ExampleService : IExampleService {
HttpContextBase _contextBase;
public ExampleService(HttpContextBase contextBase) {
_contextBase = contextBase;
}
public void DoSomething() {
var ip = HttpContext.Current.Request.UserHostAddress; <== this works
ip = _contextBase.Request.UserHostAddress; <== this fails
}
}
Global.asax
[Inject]
public IExampleService ExampleService { get; set; }
public void Application_AuthenticateRequest() {
ExampleService.DoSomething();
}
I'm missing something here, but I can't see what

Answer 1

Dependencies that are injected into classes live as long as the the class they get injected into, because the class holds a reference to them. This means that in general you should prevent injecting dependencies that are configured with a lifetime that is shorter than the containing class, since otherwise their lifetime is 'promoted' which can cause all sorts of (often hard to track) bugs.
In the case of an ASP.NET application, there is always just one HttpApplication instance that lives as long as the AppDomain lives. So what happens here is that the injected ExampleService gets promoted to one-per-appdomain (or singleton) and since the ExampleService sticks around, so does its dependency, the HttpContextBase.
The problem here of course is that an HTTP context -per definition- can't outlive a HTTP request. So you're storing a single HttpContextBase once, but it gets reused for all other requests. Fortunately ASP.NET throws an exception, otherwise you would probably be in much more trouble. Unfortunately the exception isn't very expressive. They could have done better in this case.
The solution is to not inject dependencies in your HttpApplication / MvcApplication. Ever! Although it's fine to do so when you're injecting singletons that only depend on singletons recursively, it is easy to do this wrong, and there's no verification mechanism in Ninject that signals you about this error.
Instead, always resolve IExampleService on each call to AuthenticateRequest. This ensures that you get an ExampleService with the right lifetime (hopefully configured as per-web-request or shorter) and prevents this kind of error. You can either call into the DependencyResolver class to fetch an IExampleService or call directly into the Ninject Kernel. Calling into the Kernel is fine, since the Application_AuthenticateRequest can be considered part of the Composition Root:
public void Application_AuthenticateRequest() {
var service = DependencyResolver.Current.GetService<IExampleService>();
service.DoSomething();
}