ASP.NET 4 in VB 2010 : Logging Exceptions (part 2) - Writing to the Event Log

2. Writing to the Event Log

You can interact with event logs in an ASP.NET page by using the classes in the System.Diagnostics namespace. First, import the namespace at the beginning of your code-behind file:

Imports System.Diagnostics

The following example rewrites the simple ErrorTest page to use event logging:

Public Partial Class ErrorTestLog
    Inherits System.Web.UI.Page

    Protected Sub cmdCompute_Click(ByVal sender As Object, _
      ByVal e As EventArgs) _
      Handles cmdCompute.Click

Try
            Dim A, B, Result As Decimal
            A = Decimal.Parse(txtA.Text)
            B = Decimal.Parse(txtB.Text)
            Result = A / B
            lblResult.Text = Result.ToString()
            lblResult.ForeColor = System.Drawing.Color.Black
        Catch err As Exception
            lblResult.Text = "<b>Message:</b> " & err.Message & "<br /><br />"
            lblResult.Text &= "<b>Source:</b> " & err.Source & "<br /><br />"
            lblResult.Text &= "<b>Stack Trace:</b> " & err.StackTrace
            lblResult.ForeColor = System.Drawing.Color.Red

            ' Write the information to the event log.
            Dim Log As New EventLog()
            Log.Source = "DivisionPage"
            Log.WriteEntry(err.Message, EventLogEntryType.Error)
        End Try
    End Sub

End Class

					  

The event log record will now appear in the Event Viewer utility, as shown in Figure 4. Note that logging is intended for the system administrator or developer. It doesn't replace the code you use to notify the user and explain that a problem has occurred.

Figure 4. An event record

EVENT LOG SECURITY There's a potential problem with this example. Ordinarily, Windows won't allow you to access the event log, and you'll get an exception when you attempt to run this example. The proper way to solve this problem depends on whether you're testing your web application or deploying it to a live web server. If you're just testing your logging code, the easiest option is to run Visual Studio as an administrator. To do this, right-click the Visual Studio shortcut and choose Run As Adminstrator. This step is necessary because of the user account control (UAC) safety system in Windows. UAC prevents you from using administrator privileges unless you specifically request them, all in a bid to restrict the viruses, malware, and hackers that can attack your computer. When you deploy your application to a web server, the process isn't quite as simple. Ordinarily, the account that runs your web applications has a carefully limited set of permissions. It definitely won't be an administrator account. If you want your web application to be able to use the event log, you need to give that account the permissions it needs to create event log entries. Here are the steps that you (or an administrator) must follow on the web server computer: Run regedit.exe, either by using a command-line prompt or by choosing Run from the Start menu. Browse to the HKEY_Local_Machine\SYSTEM\CurrentControlSet\Services\EventLog section of the registry. Select the EventLog folder if you want to give ASP.NET permission to all areas of the event log. Or select a specific folder that corresponds to the event log ASP.NET needs to access. Right-click the folder and choose Permissions. Add the account that ASP.NET is using to the list (or a group that this account belongs to). If you're using IIS in Windows 7, Windows Vista, or Windows Server 2008, you need to add permissions to the IIS_IUSRS group. Give the account Full Control for this section of the registry by selecting the Allow check box next to Full Control.