Dynamic HttpModule Registration in ASP.NET 4.0

net, Web Development comments edit

I came across this trick while perusing the Autofac code base with the new MVC3 integration. You no longer have to register the HttpModule that disposes of request lifetime scopes because they do it for you dynamically. Figuring out how they did it revealed two really cool little tricks I’ve not seen documentation on.

Trick #1:System.Web.PreApplicationStartMethodAttribute

.NET 4 adds a new attribute that allows you to programmatically do things just before application startup (that is, before Application_Start in your Global.asax). ASP.NET MVC3, for example, uses this hook to register build providers for the Razor view engine so you won’t have to do it manually in web.config.

To use it, first create a static class with a static method in it that contains your application startup logic. Be sure to guard against it getting called twice by having a flag indicating if it was called (sort of the way you track whether Dispose was called):

namespace MyNamespace
{
  private bool _startWasCalled = false;
  public static class PreApplicationStartCode
  {
    public static void Start()
    {
      if(!_startWasCalled)
      {
        _startWasCalled = true;
        // Do your startup logic here.
      }
    }
  }
}

You don’t have to call your class “PreApplicationStartCode,” nor do you have to call the method “Start,” but that seems to be the convention.

Once you have that class and method, mark your assembly with the attribute and point to your method:

[assembly: PreApplicationStartMethod(typeof(MyNamespace.PreApplicationStartCode), "Start")]

When the application starts, the System.Web.Hosting.HostingEnvironment.Intialize() method calls System.Web.Compilation.BuildManager.CallPreStartInitMethods() (all of that is internal, of course) and magic happens - your application startup logic runs.

Trick #2:Microsoft.Web.Infrastructure.DynamicModuleHelper.DynamicModuleUtility.RegisterModule

The Microsoft.Web.Infrastructure assembly seems to have appeared along with MVC3 and WebMatrix. The DynamicModuleUtility.RegisterModule method is a ridiculously helpful and equally ridiculously undocumented method that allows you to add an IHttpModule to the request pipeline programmatically so you don’t have to put an entry into web.config. You just pass it the type of the IHttpModule implementation and it gets added to the pipeline:

DynamicModuleUtility.RegisterModule(typeof(MyNamespace.MyHttpModule));

The only catch is… you need to call it just before application startup. (See where I’m going with this?)

Tie it together: Call DynamicModuleUtility.RegisterModule() from inside PreApplicationStartCode.Start() and you can programmatically add HttpModules to the request pipeline.

Pretty nifty, huh?

Again, I saw this first in the Autofac codebase, so props to Alex Meyer-Gleaves (who added that code to the MVC3 support in Autofac) for figuring that one out.

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