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http handler to check null requests .NET 3.0

Posted on 2009-04-28
Medium Priority
Last Modified: 2013-11-08
I have a VB .NET Web Application (.NET 3.0) which has custom session management (written by another developer).
Some times when the session objects are null the application displays the following error instead of redirecting the user to the login page.
"Object reference not set to an instance of an object".
To get rid of this message, I am thinking on creating an http handler that checks all requests looking for null values (if session object/value is null then redirect user to login page).
Unfortunatelly, I don't have experience with http handlers.
I will appreciate any references, links, code examples regarding how to build this handler.

Question by:melinalt
1 Comment

Accepted Solution

melinalt earned 0 total points
ID: 24423948
I created a class called basePageSessionExpire ("Session Expire Class" code below) .  This checks if the session is a new session. if so then redirects the user to the login page (default.aspx). Then, I changes all my pages to inherit from this class:

Inherits basePageSessionExpire

Also, for general error handling, I created a class to handle all type of errors called errorHandling (IHttpModule "Error Handling Class" code below) and added the following lines to the web.config page:

    <add name="errorHandling" type="namespace.errorHandling, namespace"/>Version=, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

This gets the error, logs it into the eventlog and send me an email.

'Session Expirte Class
Public Class basePageSessionExpire
    Inherits System.Web.UI.Page
    Public Sub New()
    End Sub
    Protected Overrides Sub OnInit(ByVal e As EventArgs)
        If (Not (Context.Session) Is Nothing) Then
            If Session.IsNewSession Then
                Dim szCookieHeader As String = Request.Headers("Cookie")
                If ((Not (szCookieHeader) Is Nothing) AndAlso (szCookieHeader.IndexOf("ASP.NET_SessionId") >= 0)) Then
                End If
            End If
        End If
    End Sub
End Class
'Error Handling Class
Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Web
Public Class errorHandling
    Implements System.Web.IHttpModule
    Public Sub Init(ByVal app As HttpApplication) Implements System.Web.IHttpModule.Init
        AddHandler app.Error, AddressOf Me.OnError
    End Sub
    Public Sub OnError(ByVal obj As Object, ByVal args As EventArgs)
        ' At this point we have information about the error
        Dim ctx As HttpContext = HttpContext.Current
        Dim exception As Exception = ctx.Server.GetLastError
        Dim errorInfo As String = ("<br><b>Offending URL:</b> " _
                    + (ctx.Request.Url.ToString + ("<br><br><b>Date/Time:</b>" & Now().ToString & "<br><br><b>Source:</b> " _
                    + (exception.Source + ("<br><br><b>Message:</b> " _
                    + (exception.Message + ("<br><br><b>Stack trace:</b> " + exception.StackTrace)))))))
        Dim SiteLogName As String = "ApplicationName"
        ' Write new error log  
        errorEmailNotification(exception, errorInfo)
        If Not EventLog.SourceExists(SiteLogName) Then
            EventLog.CreateEventSource(SiteLogName, SiteLogName)
        End If
        Dim NewLog As EventLog = New EventLog()
        NewLog.Source = SiteLogName
        NewLog.WriteEntry(exception.ToString(), EventLogEntryType.Error)
    End Sub
    Sub errorEmailNotification(ByVal exc, ByVal edetails)
        'Create a new MailMessage object and specify the"From" and "To" addresses
        Dim Email As New System.Net.Mail.MailMessage("Application Error <from@email.com>", "to@email.com")
        Email.Subject = "APPLICATION ERROR"
        Email.Body = "<span style='font-family:Verdana;font-size:9pt;'>" & edetails & "</span>"
        Email.IsBodyHtml = True
        Dim mailClient As New System.Net.Mail.SmtpClient()
        'Put your own, or your ISPs, mail server name onthis next line
        mailClient.Host = "xxx.xxx.xxx.xx"
        mailClient.UseDefaultCredentials = True
        'mailClient.Credentials = basicAuthenticationInfo
    End Sub
    Private Sub Dispose() Implements System.Web.IHttpModule.Dispose
    End Sub
End Class

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