Thread: .Net Global Error Handling

This is an example of one method to trap errors in a .Net application. This method uses the Application_Error event in the Global.ASAX and the customErrors node in the web.config file to send the user to 'friendly' error page. The actual error details and other information are sent to the email provided in the Application_Error event.

First, the Global.asax code:

Code:

<%@ Application Language="VB" %>
<%@ Import Namespace="System.Diagnostics" %>
<script runat="server">
    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs on application startup
    End Sub
    
    Sub Application_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs on application shutdown
    End Sub
        
    Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
        '//Get reference to the source of the exception chain
        Dim ex As Exception = Server.GetLastError().GetBaseException()
        Dim messageBody As New StringBuilder
        Dim errMessage As String = ex.Message.ToString()
        Dim i As Integer = 0
        
        '//Build message body
        With messageBody
            .Append("<p><b>" & Server.HtmlEncode(Request.Url.ToString) & "</b><ul>")
            For i = 1 To Request.Params.Count
                If Not Request.Params.Keys(i - 1) = "AUTH_PASSWORD" Then
                    .Append("<li>" & Server.HtmlEncode(Request.Params.Keys(i - 1).ToString) & _
                            " = <u>" + Server.HtmlEncode(Request.Params.Item(i - 1).ToString) + "</u></li>")
                End If
            Next
            .Append("</ul></p>")
            
            i = 0
            Do While (Not ex Is Nothing) And i < 10
                .Append("<hr>" & htmlException(ex))
                ex = ex.InnerException
                i += 1
            Loop
        End With
        
        '//Email details of the error to support staff
        Dim message As System.Net.Mail.MailMessage = New System.Net.Mail.MailMessage
        With message
            .From = New System.Net.Mail.MailAddress(someemail@somedomain.com)
            .To.Add(New System.Net.Mail.MailAddress(someotheremail@somedomain.com))
            .Subject = "Application Error - " & errMessage.ToString
            .IsBodyHtml = True
            .Body = messageBody.ToString()
        End With        
        
        Dim client As System.Net.Mail.SmtpClient = New System.Net.Mail.SmtpClient()
        client.Send(message)
    End Sub
    
    Function htmlException(ByVal ex As Exception) As String
        Dim str As New StringBuilder
        With str
            .Append("<p><b>Message:</b><br />" & Server.HtmlEncode(ex.Message.ToString) & "</p>")
            .Append("<p><b>Source:</b><br />" & Server.HtmlEncode(ex.Source.ToString) & "</p>")
            .Append("<p><b>Target Site:</b><br />" & Server.HtmlEncode(ex.TargetSite.Name.ToString) & "</p>")
            .Append("<p><b>Stack Trace:</b><br />" & Replace(Server.HtmlEncode(ex.StackTrace.ToString), vbCrLf, "<br />") & "</p>")
        End With
        Return str.ToString()
    End Function
    Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs when a new session is started
    End Sub
    Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs when a session ends. 
        ' Note: The Session_End event is raised only when the sessionstate mode
        ' is set to InProc in the Web.config file. If session mode is set to StateServer 
        ' or SQLServer, the event is not raised.
    End Sub
       
</script>

Then, in the web.config:

Code:

<customErrors mode="RemoteOnly" defaultRedirect="~/Error.aspx">
 <error statusCode="404" redirect="~/Error.aspx?code=404"/>
 <error statusCode="403" redirect="~/Error.aspx?code=403"/>
</customErrors>

This allows you to determine which page the user gets sent to. Notice the <error statusCode /> node. You can leave this blank or put as many statusCodes as you need here. Also, note the mode attribute. I like to use RemoteOnly as it allows anyone who is physically at the server to see the actual error message, but sends anyone who is logged in remotely to the error page. The other options are "On" and "Off".

On Error.aspx, you can put a generic error message, or use a Select Case based on the status code passed in the 'code' querystring value, to determine what error message to display to the user.

This is just one method to handle errors in your application.

Is it possible to show an error message on the page rather than redirecting to another page and dependant on the action that was being taken at the time?

E.g. if it was whilst loading a specific item on the page, like a dropdownlist from a List(of ) object, or if it was whilst trying to update a particular db entry.

Can this be used in conjunction with a Try..Catch block, so the try..catch outputs a message to the screen whilst the Application_Error Sub sends an email detailing the error?

You can capture the errors using Try...Catch and then Rethrow the error so that the Application_Error event is fired. You would turn off customErrors in the web.config for this to work.

Originally Posted by jmurrayhead

You can capture the errors using Try...Catch and then Rethrow the error so that the Application_Error event is fired. You would turn off customErrors in the web.config for this to work.

So if I had a Try...Catch block in a DAL function then I could use:-

Code:

Catch ex As Exception
myObj.err = ex.toString
Throw New Exception

And it would still return to my page so I could render a message to the screen on the same page? and then Application_Error sub would send me the error email?

To rethrow the exception, I believe it's

Code:

Catch ex As Exception
'Do Whatever
Throw