In this article I am going to share some techniques on globally trapping and handling errors that occur in a web application and then how to notify administrators/developers that the error has occurred. I will use the Application_OnError subroutine of the global.asax file to trap the errors and then the aspNetEmail component to send the error notification e-mails.[bold]Introduction[/bold]When porting our campus? student information portal to ASP.NET, we realized that the previous ASP ...
Contributed by Tyler Peelen Rating: / 2 August 21, 2002
In this article I am going to share some techniques on globally trapping and handling errors that occur in a web application and then how to notify administrators/developers that the error has occurred. I will use the Application_OnError subroutine of the global.asax file to trap the errors and then the aspNetEmail component to send the error notification e-mails.[bold]Introduction[/bold]When porting our campus? student information portal to ASP.NET, we realized that the previous ASP version generated many errors that we had no idea had occurred. It was decided that in the new version we needed some way to trap errors and then e-mail us information about the error.We didn?t want to use System.Web.Mail because our administrators did not want to install SMTP server on all of our web servers. After looking around a bit, we decided to go with the aspNetEmail component, which in addition to allowing us to use our single campus SMTP server, also allowed for us to dynamically create the HTML error e-mails and include DataGrids.[bold]Error Trapping[/bold]Error trapping and handling at an application level is super easy to do in ASP.NET. All errors occur at the application level, so you can trap errors using the Application_OnError method of global.asax.In our Application_OnError subroutine, we grab the exception for the error using Context.Error.GetBaseException() as seen in the code below:
Sub Application_Error
(ByVal sender As Object, ByVal e As EventArgs) Dim ex As Exception = Context.Error.GetBaseException()
Dim objErrHandler As New ErrorHandler() objErrHandler.SendHtmlErrorEmail(ex) End Sub
After the exception is grabbed, it is passed to the SendHtmlErrorEmail function of the ErrorHandler class, which does the work of generating the e-mail.[bold]Collecting additional information[/bold]In addition to being sent information about the error itself, we also needed to be sent information about the session that generated the error, like QueryString & Form variables, Server Variables and Cookies. This kind of information helps tremendously in the bug-tracking process.We wanted to get the variables into a DataTable so that later we could easily bind it to a DataGrid. The variables are either stored in a NameValueCollection or an HttpCookiesCollection, so I wrote a couple of functions that iterate through the collections and add rows to a datatable:
Private Function GetVariables(ByVal inputVarCollection As System.Collections.Specialized.NameValueCollection) As DataTable Dim objTable As New DataTable() objTable.Columns.Add("Name", GetType(String)) objTable.Columns.Add("Value", GetType(String))
Dim strItem As String Dim objRow As DataRow For Each strItem In inputVarCollection objRow = objTable.NewRow() objRow("Name") = strItem objRow("Value") = inputVarCollection(strItem) objTable.Rows.Add(objRow) Next
Return objTable End Function
Private Function GetCookieVars() As DataTable Dim objTable As New DataTable() objTable.Columns.Add("Name", GetType(String)) objTable.Columns.Add("Value", GetType(String))
Dim strItem As String Dim objRow As DataRow For Each strItem In HttpContext.Current.Request.Cookies objRow = objTable.NewRow() objRow("Name") = strItem objRow("Value") = HttpContext.Current.Request.Cookies(strItem).Value objTable.Rows.Add(objRow) Next
Return objTable End Function
[bold]Creating the DataGrids[/bold]I fashioned the error e-mail after the output generated when you use ASP.NET tracing. The easiest way to do this was to use DataGrids, but since they were all going to be the same design and they needed to be programmatically generated on the fly, I wrote a method that set up the templates for the grid:
Private Function CreateDataGrid() As System.Web.UI.WebControls.DataGrid
Dim objDataGrid As New DataGrid() objDataGrid.Width = System.Web.UI.WebControls.Unit.Percentage(100) objDataGrid.BorderWidth = System.Web.UI.WebControls.Unit.Pixel(0) objDataGrid.CellPadding = "2"
[bold]Creating the e-mail[/bold]This is the most complicated step of the process and one of the main reasons that we went with the aspNetEmail component. System.Web.Mail didn?t allow us to add controls (like DataGrid) to an e-mail message, so we would have had to build a lot of ugly ASP-like code to generate nice, uniform looking tables. Why reinvent the wheel when there are server controls that do the dirty work for you? We were able to add the DataGrid controls to our e-mail messages using aspNetEmail?s AppendControlToBody function. The code below will add a DataGrid containing cookie information and send the e-mail:
Dim objMessage
As New EmailMessage("your smtp server")
Dim objCookiesDG As DataGrid = CreateDataGrid() objCookiesDG.DataSource = GetCookieVars() objCookiesDG.DataBind() objMessage.AppendControlToBody(objCookiesDG)
[bold]Conclusion[/bold]It is super easy to globally handle and trap errors at an application level, and with a couple of steps, you can create e-mail notification of those errors in a standard, easy to read format. You can view the entire ErrorHandlerClass below:
Imports System
.Text Imports aspNetEmail
Public Class ErrorHandler
Private Function CreateDataGrid() As System.Web.UI.WebControls.DataGrid
Dim objDataGrid As New DataGrid() objDataGrid.Width = System.Web.UI.WebControls.Unit.Percentage(100) objDataGrid.BorderWidth = System.Web.UI.WebControls.Unit.Pixel(0) objDataGrid.CellPadding = "2"
Private Function GetVariables(ByVal inputVarCollection As System.Collections.Specialized.NameValueCollection) As DataTable Dim objTable As New DataTable() objTable.Columns.Add("Name", GetType(String)) objTable.Columns.Add("Value", GetType(String))
Dim strItem As String Dim objRow As DataRow For Each strItem In inputVarCollection objRow = objTable.NewRow() objRow("Name") = strItem objRow("Value") = inputVarCollection(strItem) objTable.Rows.Add(objRow) Next
Return objTable End Function
Private Function GetCookieVars() As DataTable Dim objTable As New DataTable() objTable.Columns.Add("Name", GetType(String)) objTable.Columns.Add("Value", GetType(String))
Dim strItem As String Dim objRow As DataRow For Each strItem In HttpContext.Current.Request.Cookies objRow = objTable.NewRow() objRow("Name") = strItem objRow("Value") = HttpContext.Current.Request.Cookies(strItem).Value objTable.Rows.Add(objRow) Next
Return objTable End Function
Public Sub SendHtmlErrorEmail(ByVal ex As Exception) Dim objMessage As New EmailMessage("your smtp server")
[bold]About the Author[/bold]Tyler Peelen is the lead ASP.NET developer for the University of Wisconsin ? Stevens Point, my.uwsp.edu portal project. Although he is only 18, he has a firm grasp of ASP.NET has been programming for 10 years.
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