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Introducing HTTPModules
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Introduction
Introduction

In The life of an ASP.NET Request I looked at the pipeline that gets the request to the appropriate handlers. In the diagram I showed that further up the pipeline there were HTTPModules that allowed examination and modification of the HTTPApplication object as it got passed down. This article is going to take a more in-depth look at the HTTPModule and creation of one.

The ASP.NET Page Flow and HTTPModules

The ASP.NET Flow and HTTPModule

When the request gets sent to the HTTPApplication object, it then gets sent to any HTTPModules that are registered in the web.config file of the application.

There are already HTTPModules in ASP.NET that are a part of every application. These include authentication, authorization and session state modules and if you've ever handled the Session_Start or Session_End events in global.asax then you have used these modules (specifically the session state module).

Why use an HTTPModule

HTTPModules can take the request and do anything to it before a Page object is created and used. For example, if you only wanted people using Netscape to access your pages, then you could write an HTTPModule that executed before a Page was created to block other browsers. You could also use it to monitor performance counters or calculate the elapsed time for a request.

What is an HTTPModule

An HTTPModule qualifies as anything that implements the IHTTPModule interface. The interface only has two methods -

Public Interface IHttpModule

Public Sub Dispose()
Public Sub Init(context as HttpApplication)

End Interface
Creating an HTTPModule
Creation of an HTTPModule

Below is just a simple HTTPModule -

Imports System
Imports
System.Web
Imports
System.Diagnostics

Public Class SampleModule : Implements IHttpModule

Public Sub Dispose() Implements IHttpModule.Dispose
End
Sub

Public Sub Init(ByVal application As HttpApplication) Implements IHttpModule.Init
AddHandler
application.BeginRequest, AddressOf Me
.OnBeginRequest
End
Sub

Public Sub OnBeginRequest(ByVal sender As Object, ByVal e As EventArgs)
Dim
app As HttpApplication = CType(sender, HttpApplication)
Dim
browscap As
HttpBrowserCapabilities = app.Request.Browser
If
browscap.Browser = "Netscape"
Then
'w00t
ElseIf browscap.Browser = "IE" Then
'EEK!
Else
'wha?
End If
End Sub

End Class

As you can see, this class implements all of the required methods, it also adds a handler for one of it's events. The event handling is usually the way that you time your events correctly as shoving it in the Init method isn't the most effective way.

Currently it doesn't do anything but check the browser and print the name of the browser to the screen.

The next thing you have to do is add it to the web.config of the application like -

<httpModule>
<add name="SampleModle" type="HttpModHan.SampleModule, HttpModHan" />
</httpModule>

<add name="A Name" type="Class, Assembly" />

It should look like -

When running.

Events in HTTPModules
Events in HTTPModules

Now we get to add some functionality to our module.

Imports System
Imports
System.Web

Public
Class SampleModule : Implements IHttpModule
Public
Event bdet As
bdetEventHandler
Public
Delegate Sub bdetEventHandler(ByVal sender As Object, ByVal e As
BrowserEventArgs)

Public Sub Dispose() Implements IHttpModule.Dispose
End
Sub

Public Sub Init(ByVal application As HttpApplication) Implements IHttpModule.Init
AddHandler
application.BeginRequest, AddressOf Me
.OnBeginRequest
End
Sub

Public Sub OnBeginRequest(ByVal sender As Object, ByVal e As EventArgs)
Dim
app As HttpApplication = CType(sender, HttpApplication)
Dim
browscap As
HttpBrowserCapabilities = app.Request.Browser
If
browscap.Browser = "Netscape"
Then
RaiseEvent bdet(Me, New BrowserEventArgs("Netscape"))
ElseIf
browscap.Browser = "IE"
Then
RaiseEvent bdet(Me, New BrowserEventArgs("Internet Explorer"))
Else
RaiseEvent bdet(Me, New BrowserEventArgs("another browser"))
End
If
End Sub
End
Class

<Serializable()> Public Class BrowserEventArgs : Inherits EventArgs
Private
bname As
String

Public
Property BrowserName() As
String
Get
Return
bname
End
Get
Set(ByVal Value As String)
bname = Value

End
Set
End Property

Public Sub New(ByVal BrowserID As String)
BrowserName = BrowserID

End
Sub

End
Class

I've added a public event to the code called bdet and added a delegate for it's custom signature (using the BrowserEventArgs instead of EventArgs, see the Related Articles for more information on this) and then raising the event with the appropriate browser name.

Then in web.config

<httpModule>
<add name="SampMod" type="HttpModHan.SampleModule, HttpModHan" />
</httpModule>

And finally in Global.asax -

Imports HttpModHan

Sub SampMod_bdet(ByVal sender As Object, ByVal e As BrowserEventArgs)
Response.Write("You are using " & e.BrowserName)

End
Sub

And will look something like -

When running.

Summary

Well, this article should have taught you the basics of setting up an HTTPModule and adding some events to it to make it even more useful.


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