The Two Interceptors: HttpModule and HttpHandlers

http://www.codeproject.com/Articles/30907/The-Two-Interceptors-HttpModule-and-HttpHandlers

Table of Contents

Introduction
The Problem
HttpHandler - The Extension Based Preprocessor
HttpModule- The Event Based Preprocessor
The Overall Picture of Handler and Modules
Steps to Implement HttpHandlers
Steps to Implement HttpModule
The Final Output
Reference

Introduction

Many times we want to implement pre-processing logic before a request hits the IIS resources. For instance you would like to apply security mechanism, URL rewriting, filter something in the request, etc. ASP.NET has provided two types
of interception

HttpModule

and

HttpHandler

. This article walks through
it.
For the last few days, I have been writing and recording videos in design patterns, UML, FPA, Enterprise blocks and lot more. You can watch the videos here.
You can download my 400 .NET FAQ EBook from here.

The Problem

Many times we need to inject some kind of logic before the page is requested. Some of the commonly used pre-processing logics are stat counters, URL rewriting, authentication / authorization and many more. We can do this in the code
behind but then that can lead to lot of complication and tangled code. The code behind will not solve the purpose because in some implementations like authorization, we want the logic to execute before it reaches the resource. ASP.NET provides two ways of
injecting logic in the request pipeline

HttpHandlers

and

HttpModules

.

HttpHandler - The Extension Based Preprocessor

HttpHandler

help us to inject pre-processing logic based on the extension of the file name requested. So when a page is requested,

HttpHandler

executes
on the base of extension file names and on the base of verbs. For instance, you can visualize from the figure below how we have different handlers mapped to file extension. We can also map one handler to multiple file extensions. For instance, when any client
requests for file with extension ‘GIF’ and ‘JPEG’,

handler3

pre-processing logic executes.

HttpModule- The Event Based Preprocessor

HttpModule

is an event based methodology to inject pre-processing logic before any resource is requested. When any client sends a request
for a resource, the request pipeline emits a lot of events as shown in the figure below:



Below is a detailed explanation of the events. We have just pasted this from here.

BeginRequest

: Request has been started. If you need to do something at the beginning of a request (for example, display advertisement
banners at the top of each page), synchronize this event.

AuthenticateRequest

: If you want to plug in your own custom authentication scheme (for example, look up a user against a database to validate
the password), build a module that synchronizes this event and authenticates the user in a way that you want to.

AuthorizeRequest

: This event is used internally to implement authorization mechanisms (for example, to store your access control lists
(ACLs) in a database rather than in the file system). Although you can override this event, there are not many good reasons to do so.

PreRequestHandlerExecute

: This event occurs before the HTTP handler is executed.

PostRequestHandlerExecute

: This event occurs after the HTTP handler is executed.

EndRequest

: Request has been completed. You may want to build a debugging module that gathers information throughout the request and then
writes the information to the page.

We can register these events with the

HttpModules

. So when the request pipe line executes depending on the event registered, the logic
from the modules is processed.

The Overall Picture of Handler and Modules

Now that we have gone through the basics, let's understand what is the Microsoft definition for handler and modules to get the overall picture.
Reference: INFO: ASP.NET HTTP Modules and HTTP Handlers Overview
“Modules are called before and after the handler executes. Modules enable developers to intercept, participate in, or modify each individual request. Handlers are used to process individual endpoint requests. Handlers enable the ASP.NET
Framework to process individual HTTP URLs or groups of URL extensions within an application. Unlike modules, only one handler is used to process a request”.

Steps to Implement HttpHandlers

Step 1

HttpHandlers

are nothing but classes which have pre-processing logic implemented. So the first thing is to create a class project and reference

System.Web

namespace
and implement the

IHttpHandler

interface as shown in the below code snippet.

IHttpHandler

interface
has two methods which needs to be implemented; one is the

ProcessRequest

and the other is the

IsResuable

.
In the

ProcessRequest

method, we are just entering the URL into the file and displaying the same into the browser. We have manipulated the context response object
to send the display to the browser.


Collapse | Copy
Code

using System;
using System.Web;
using System.IO;
namespace MyPipeLine
{
public class clsMyHandler : IHttpHandler
{
public void ProcessRequest(System.Web.HttpContext context)
{
context.Response.Write("The page request is " + context.Request.RawUrl.ToString());
StreamWriter sw = new StreamWriter(@"C:\requestLog.txt",true);
sw.WriteLine("Page requested at " + DateTime.Now.ToString() +
context.Request.RawUrl); sw.Close();
}
public bool IsReusable
{
get
{
return true;
}
}
}

Step 2

In step 2, we need to make an entry of

HttpHandlers

tag. In the tag, we need to specify which kind of extension requested will invoke our
class.


Collapse | Copy
Code

<system.web>
<httpHandlers>
<add verb="*" path="*.Shiv,*.Koirala" type="MyPipeLine.clsMyHandler, MyPipeLine"/>
</httpHandlers>
</system.web>

Once done, request for page name with extension ‘Shiv’ and you should see a display as shown below. So what has happened is when the IIS sees that request is for a ‘.shiv’ page extension, it just calls the

clsMyHandler

class
pre-processing logic.

Steps to Implement HttpModule

Step 1

As discussed previously,

HttpModule

is an event pre-processor. So the first thing is to implement the

IHttpModule

and
register the necessary events which this module should subscribe. For instance, we have registered in this sample for

BeginRequest

and

EndRequest

events.
In those events, we have just written an entry on to the log file.


Collapse | Copy
Code

public class clsMyModule : IHttpModule
{
public clsMyModule()
{}
public void Init(HttpApplication objApplication)
{
// Register event handler of the pipe line
objApplication.BeginRequest += new EventHandler(this.context_BeginRequest);
objApplication.EndRequest += new EventHandler(this.context_EndRequest);
}
public void Dispose()
{
}
public void context_EndRequest(object sender, EventArgs e)
{
StreamWriter sw = new StreamWriter(@"C:\requestLog.txt",true);
sw.WriteLine("End Request called at " + DateTime.Now.ToString()); sw.Close();
}
public void context_BeginRequest(object sender, EventArgs e)
{
StreamWriter sw = new StreamWriter(@"C:\requestLog.txt",true);
sw.WriteLine("Begin request called at " + DateTime.Now.ToString()); sw.Close();
}
}

Step 2

We need to enter those module entries into the

HttpModule

tag as shown in the below code snippet:


Collapse | Copy
Code

<httpModules>
<add name="clsMyModule" type="MyPipeLine.clsMyModule, MyPipeLine"/>
</httpModules>

The Final Output

If you run the code, you should see something like this in the RequestLog.txt. The above example is not so practical. But it will help us understand the fundamentals.


Collapse | Copy
Code

Begin request called at 11/12/2008 6:32:00 PM
End Request called at 11/12/2008 6:32:00 PM
Begin request called at 11/12/2008 6:32:03 PM
End Request called at 11/12/2008 6:32:03 PM
Begin request called at 11/12/2008 6:32:06 PM
End Request called at 11/12/2008 6:32:06 PM
Begin request called at 11/12/2008 8:36:04 PM
End Request called at 11/12/2008 8:36:04 PM
Begin request called at 11/12/2008 8:37:06 PM
End Request called at 11/12/2008 8:37:06 PM
Begin request called at 11/12/2008 8:37:09 PM
End Request called at 11/12/2008 8:37:09 PM
Begin request called at 11/12/2008 8:37:38 PM
Page requested at 11/12/2008 8:37:38 PM/WebSiteHandlerDemo/Articles.shiv
End Request called at 11/12/2008 8:37:38 PM

Reference

INFO: ASP.NET HTTP Modules and HTTP Handlers Overview

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Shivprasad koirala