Using custom errors and write errors to the Event log

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July 14, 2001

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Complete example using custom errors and write errors to the Event log

Writing to the Windows 2000 Event Log is a powerful feature of the ASP.NET and .NET framework. For those individuals who work in a large company and want to make track application errors, writing to the event log is a must!

There wasn't any complete demos actually showing from A - Z how this was setup for the novice/common developer like me. Most novice developers need to be spoon fed how things are done the first time, once they see a simple example, they'll understand how the process works! This example is my reference how to make a complete application from A - Z setup and fail to understand how it works.

Step 1

First of all the steps I followed was to create a simple web called eventlog. I also created the application rootso the global.asax file would fire.

Step 2

I opened the global web.config file and turned on custom errors. Path to this is c:winntmicrosoft.netframework.. There are 3 choices available currently On, Off and RemoteOnly. From attending the conference, the recommended was RemoteOnly. This means anyone not on the console of the machine will see a friendly error and not the real thing. For this example I chose On. You also could leave the global web.config file custom errors turned off and configure at application level's web.config Either way works just fine.

web.config -- this file is placed in the root of the eventlog application root.

Step 3

The next few items are just to create sample pages to make the application complete. I created a web.config, global.asax, Default.aspx page, and three sample error pages. 404page.aspx, 403page.aspx and customerrorpage.aspx page.

Here are those pages code for all pages.

Global.asax Page - This uses the Application_OnError event to capture stuff if an error happens

<%@ Import Namespace="System" %>
<%@ Import Namespace="System.Diagnostics" %>
<script language="VB" runat=server>

Public Sub Application_OnError(Sender as Object, E as EventArgs)
'Captures the error and converts to a string
dim LastError as Exception = Server.GetLastError()
Dim ErrMessage as String = LastError.toString()

Dim LogName As String = "MyLog"
Dim Message As String = "Url " & Request.Path & " Error: " & ErrMessage

' Create Event Log if It Doesn't Exist
If (Not EventLog.SourceExists(LogName)) Then
EventLog.CreateEventSource(LogName, LogName)
End if

Dim Log as New EventLog
Log.Source = LogName

'These are the five options that will display a different icon.
'The numbers are just to show the order. These aren't required
Log.WriteEntry(Message, EventLogEntryType.Information, 1)
' Log.WriteEntry(Message, EventLogEntryType.Error, 2)
' Log.WriteEntry(Message, EventLogEntryType.Warning, 3)
' Log.WriteEntry(Message, EventLogEntryType.SuccessAudit, 4)
' Log.WriteEntry(Message, EventLogEntryType.FailureAudit, 5)
End Sub
</script>

Default.aspx page
<% @Language="VB" %>
<script language="VB" runat=server>
Sub Page_Load(Sender As Object, E As EventArgs)
If IsPostBack Then
'Declare all variables
dim x as integer
dim y as integer
dim z as integer

'set x and y to values to be divided by zero
x = 1
y = 0

'perform the division by zero to raise the error
z = x/y
End Sub
</script>

Customerrorpage.aspx

<html>
<head></head>
<body>
<h1>custom error page</h1>
</body>
</html>

404page.aspx --Capture all 404(Not Found pages)

<html>
<head></head>
<body>
<h1>404 error page</h1>
</body>
</html>

403page.aspx --Capture all 403(Restricted pages)

<html>
<head></head>
<body>
<h1>403 error page</h1>
</body>
</html>

Step 4

After all webs are created, web.config files in place. It was time to test out the application to see if it works. Type in http://localhost/eventlog/default.aspx file, this will display a button. Click it and see if this actually creates the log and writes the information to the event log. Actually only the custom error page will be displayed, the eventlog.aspx page will error and be transfered to the customerrorpage.aspx. The URL will be something like this.

http://localhost/eventlog/customerrorpage.aspx?aspxerrorpath=/eventlog/eventlog.aspx

The page will appear like this

Step 5.

Verify the log was created and entry was placed in that log. The example shows a Critical error message. You also can show informational or yellow warning messages. For this example we showed all 5 possible entries

Open the Error and view the message. If the customerror wasn't turned on, this error would have shown up in the browser instead of the eventlog. Not pretty!!

Thats it! This was a high-level example with examples but hopefully helps in understanding how a sample application and using the new Error-handling features of ASP.NET... Enjoy!!

Parameters for WriteEntry Method

source: The source by which the application is registered on the specified computer.
message: The string to write to the event log.
type: One of the EventLogEntryType values.
eventID: The application-specific identifier for the event.
category: The application-specific subcategory associated with the message.
rawData: An array of bytes that holds the binary data associated with the entry.

Five possible types of Event log messages. These show the different types of icons

Member Name	Description
Error	An error event. This indicates a significant problem the user should know about; usually a loss of functionality or data.
FailureAudit	A failure audit event. This indicates a security event that occurs when an audited access attempt fails; for example, a failed attempt to open a file.
Information	An information event. This indicates a significant, successful operation.
SuccessAudit	A success audit event. This indicates a security event that occurs when an audited access attempt is successful; for example, logging on successfully.
Warning	A warning event. This indicates a problem that is not immediately significant, but that may signify conditions that could cause future problems.