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 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

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.

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.