Installing an HttpModule via Dropping in the bin without Editing Web.config / by Matt Wrock

Last week another team wanted to use an HttpModule I had written. The problem is that they had no access to the hosting platform of their content. Their DLLs are consumed via MEF by a rather large infrastructure affectionately known as MTPS or what the world knows as the MSDN Library.

Unfortunately, the functionality provided by this DLL requires that it be run as an HttpModule. So I remembered reading a post of David Ebbo that described how to load an HttpModule dynamically at run time.The topic suddenly seemed relevant to this issue so I revisited his blog and applied his instructions to my assembly and wow…sure enough, just drop my dll in the bin and BAM! it loads and runs. If you would like to see a very small example app that does this please see my code sample on the MSDN Samples Gallery.

Technically, to accomplish this the code involved couldn’t be more trivial. Here is a quick walk through.

Specify a PreApplicationStartMethod

In the of the assembly containing the HttpModule, the below attribute must be included:

[assembly: PreApplicationStartMethod(typeof(Loader), "LoadModule")]

This line points to a Method inside the assembly which should be envoked during Pre Application Startup. This event occurs even before the Application Start event in the ASP.NET lifecycle. The official MSDN Library reference for this attribute can be found here. Of course this is assuming that there is a class either inside the assembly or referencable to the assembly of type Loader and that it contains a method called "LoadModule."

Dynamically Register the Module

Inside the LoadModule method, you will use the Microsoft.Web.Infrastructure assembly to actually register the module. The Microsoft.Web.Infrastructure assembly is not included in the .net BCL but you can easily download and install via Nuget. Here is a look at the entire Loader class:

using Microsoft.Web.Infrastructure.DynamicModuleHelper; 

namespace HttpModule {     public class Loader     {         public static void LoadModule()         {             DynamicModuleUtility.RegisterModule(typeof (DynamicModule));         }     } } 
As you can see there is not a whole lot to this. Just one line simply tells the run time the type name of the HttpModule to be registered. Here the HttpModule is inside a class called DynamicModule. The type that you pass to RegisterModule must derrive from IHttpModule.

Is this a good practice?

At first glance this seems really cool. It removes one setup step that consumers of the HttpModule would normally have to go through to install a third party module. It appears that this would provide a more friction free installation story for any third party module. What can be bad about that?

Well maybe I'm over thinking this but my first concern here is discoverability. What if the module has heavy hitting impact on the markup of the page. I envision a new developer or team inheriting such an application and wonder just how long it will take for them to find where this "alteration" is coming from especially in a sophisticated application with perhaps 20 referenced DLLs and lots of internal code. Or perhaps a team member drops in such a module and forgets to tell the team she put it there. I'm thinking that at some point in this story some negative energy will be exerted. Perhaps even a tear shed?

As an author myself of an open source HttpModule that can transform the markup of a site, this is of particular interest to me. My RequestReduce project is a module that combines and minifies CSS and Javascript and sprites and optimizes CSS background images on the fly. Since the default behavior of the module simply requires the module to be registered in the user's web.confg, if I were to employ the technique used in this sample, the module installation would have a truly plug and play installation story which sounds very awesome and I am tempted to add it. But I worry.

I'd be interested in others feedback in this matter. Do you think it is a good idea in general practice? Do you think the perhaps limited discoverability poses a true risk? In the end, do you think consumers would be more or less attracted to such an installation story?