Application_Error ignored?

Question

I've put the following code in global.asax

<%@ Application Language="VB" %>

<script runat="server">

    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code, der beim Starten der Anwendung ausgeführt wird.
    End Sub


    Sub Application_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Code, der beim Beenden der Anwendung ausgeführt wird.
    End Sub

    ' HelpLink Gets or sets a link to the help file associated with this exception. 
    ' InnerException Gets the Exception instance that caused the current exception.  
    ' Message Gets a message that describes the current exception.  
    ' Source Gets or sets the name of the application or the object that causes the error.  
    ' StackTrace Gets a string representation of the frames on the call stack at the time the current exception was thrown.  
    ' TargetSite Gets the method that throws the current exception.  

    ' http://www.developer.com/net/asp/article.php/961301/Global-Exception-Handling-with-ASPNET.htm
    Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
        ' Code, der bei einem nicht behandelten Fehler ausgeführt wird.
        'get reference to the source of the exception chain
        Dim ex As Exception = Server.GetLastError().GetBaseException()

        'log the details of the exception and page state to the
        'Windows 2000 Event Log
        'System.Diagnostics.EventLog.WriteEntry("source", "MESSAGE: " + ex.Message + "\nSOURCE: " + ex.Source + "\nFORM: " + _
        'HttpContext.Current.Request.Form.ToString() + "\nQUERYSTRING: " + HttpContext.Current.Request.QueryString.ToString() + _
        '"\nTARGETSITE: " + ex.TargetSite.ToString + "\nSTACKTRACE: " + ex.StackTrace, System.Diagnostics.EventLogEntryType.Error)

        Response.Redirect("GeneralError.aspx", False)
        'Response.Redirect("GeneralError.aspx")
        'Insert optional email notification here...

        ''this is what we are sending
        'Dim post_data As String = "MESSAGE: " + ex.Message + "\nSOURCE: " + ex.Source + "\nFORM: " + _
        'HttpContext.Current.Request.Form.ToString() + "\nQUERYSTRING: " + HttpContext.Current.Request.QueryString.ToString() + _
        '"\nTARGETSITE: " + ex.TargetSite.ToString + "\nSTACKTRACE: " + ex.StackTrace
        'post_data = HttpContext.Current.Server.UrlEncode(post_data)

        '' this is where we will send it
        'Dim uri As String = "http://localhost/cor_raumplaner/GlobalError.aspx"

        '' create a request
        'Dim request As System.Net.HttpWebRequest = DirectCast(System.Net.WebRequest.Create(uri), System.Net.HttpWebRequest)
        'request.KeepAlive = False
        'request.ProtocolVersion = System.Net.HttpVersion.Version10
        'request.Method = "POST"

        '' turn our request string into a byte stream
        'Dim postBytes As Byte() = Encoding.ASCII.GetBytes(post_data)

        '' this is important - make sure you specify type this way
        'request.ContentType = "application/x-www-form-urlencoded"
        'request.ContentLength = postBytes.Length
        'Dim requestStream As System.IO.Stream = request.GetRequestStream()

        '' now send it
        'requestStream.Write(postBytes, 0, postBytes.Length)
        'requestStream.Close()

        '' grab te response and print it out to the console along with the status code
        'Dim response1 As System.Net.HttpWebResponse = DirectCast(request.GetResponse(), System.Net.HttpWebResponse)

        'Response.Write(New System.IO.StreamReader(response1.GetResponseStream()).ReadToEnd())
        'Response.Write(response1.StatusCode)
    End Sub

    Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code, der beim Starten einer neuen Sitzung ausgeführt wird.
    End Sub

    Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Code, der am Ende einer Sitzung ausgeführt wird. 
        ' Hinweis: Das Session_End-Ereignis wird nur ausgelöst, wenn der sessionstate-Modus
        ' in der Datei "Web.config" auf InProc festgelegt wird. Wenn der Sitzungsmodus auf StateServer 
        ' oder SQLServer festgelegt wird, wird das Ereignis nicht ausgelöst.
    End Sub

</script>

Then i've added a new page, and made it generate an error, like this:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Dim mylanguage As String = Session("Language")
    Dim iLength As Integer = mylanguage.Length

End Sub

Now, the problem is seems to be that the application error handler in global.asax doesn't get called, instead I receive a standard exception: "Object reference not set to an instance of an object"

Why ? Shouldn't it call the error handler and redirect to the GeneralError.aspx page ?

Answer 1

In your web.config file make sure you have Debug=false If it's turned on you'll see all the error messages and your error handler may not work as intended.

Answer 2

When using the AspNetDevelopmentServer shipped with Visual Studio it´ll ignore if the error isn't cleared before continuing outside the Application_Error but in IIS the error is still in the session when exiting the method and thus it´s deemed unhandled by IIS which´ll then display it in its own error page.

To correct this use Server.ClearError() after Server.GetLastError() to make sure you signal the server that You will handle the error.

/ Regards