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Managing Unhandled Exceptions

Managed applications (that is, applications that use the services of the CLR) execute in an isolated environment called an AppDomain. The AppDomain class provides a set of events that allows you to respond when an assembly is loaded, when an application domain is about to be unloaded, or when an application throws an unhandled exception. I'll discuss the AppDomain class in more detail in Chapter 4, "Creating and Managing Components and Assemblies," but for now I'll use its UnhandledException event. This event fires when no other exception handler catches an exception. The CLR passes this event an instance of the UnhandledExceptionArgs class. The properties of the UnhandledExceptionEventArgs class are listed in Table 3.2.

Table 3.2 Important Properties of the UnhandledExceptionEventArgs Class




It gets the unhandled exception object corresponding to the current domain.


It has a Boolean value that indicates whether the Common Language Runtime is terminating.

You can attach an event handler to this event to take custom actions such as logging the exception-related information. A log maintained over a period can help analyze and find patterns that can give you useful debugging information. You can log the information related to an event in several ways:

  • Windows event log

  • Custom log files

  • Databases such as SQL Server 2000

  • Email notifications

Among these ways, the Windows event log offers a very robust way of event logging. It requires the minimum infrastructure for logging the event. The other cases are not as fail-safe because your application can lose connectivity with a database or with the SMTP server, because or you might have problems writing an entry in a custom log file.

The .NET Framework provides access to the Windows event log through the EventLog class. Windows 2000 (and later versions) has three default logs: Application, System, and Security. You can use the EventLog class to create custom event logs. These event logs can be easily viewed using the Windows Event Viewer utility.

You should know about the important members of the EventLog class listed in Table 3.3. See Step By Step 3.5 to use the event log to log unhandled exceptions.


When Not to Use the Event Log The Windows event log is not available on older version of Windows, such as Windows 98. If your application needs to support those computers, you might want to create a custom error log in a file. Also, in case of a distributed application, you might want to log all events centrally in a SQL Server database. To make the scheme more robust, you could choose to log locally if a database is not available and transfer the log to the central database when it is available again.

Table 3.3 Important Members of the EventLog Class






Removes all entries from the event log to make it empty



Creates an event source that you can use to write to a standard or custom event log



Gets the contents of the event log



The name of the log to read from or write to



The name of the computer on which to read or write events



The event source name to register and use when writing to the event log



Specifies whether the event source exists on a computer



Writes an entry in the event log

STEP BY STEP 3.5 Logging Unhandled Exceptions in the Windows Event Log

  1. Add a new Windows Form to your Visual Basic .NET project.

  2. Place three TextBox controls (txtMiles, txtGallons, and txtEfficiency) and a Button (btnCalculate) on the form's surface and arrange them as was shown in Figure 3.1. Add the label controls as necessary.

  3. Double-click the Button control and add the following code to handle its Click event:

    Private Sub btnCalculate_Click( _
     ByVal sender As System.Object, _
     ByVal e As System.EventArgs) _
     Handles btnCalculate.Click
      ' This code has no error checking. If something
      ' goes wrong at runtime, it will throw an exception
      Dim decMiles As Decimal = Convert.ToDecimal( _
      Dim decGallons As Decimal = Convert.ToDecimal( _
      Dim decEfficiency As Decimal = _
      decMiles / decGallons
      txtEfficiency.Text = String.Format( _
      "{0:n}", decEfficiency)
    End Sub
  4. Add the following event handler to the end of the form's module:

    Private Sub UnhandledExceptionHandler( _
     ByVal sender As Object, _
     ByVal ue As UnhandledExceptionEventArgs)
      Dim unhandledException As Exception = +
       CType(ue.ExceptionObject, Exception)
      ' If no event source exist, create an event source
      If Not EventLog.SourceExists( _
       "Mileage Efficiency Calculator") Then
        EventLog.CreateEventSource( _
         "Mileage Efficiency Calculator", _
         "Mileage Efficiency Calculator Log")
      End If
      ' Create an EventLog instance and assign its source.
      Dim el As EventLog = New EventLog()
      el.Source = "Mileage Efficiency Calculator"
      ' Write an informational entry to the event log.  
      MessageBox.Show( _
       "An exception occurred: Created an entry in the log file")

    End Sub

  5. Modify the form's New method as follows:

    Public Sub New()
      'This call is required by the Windows Form Designer.
      ' Add any initialization after the 
      ' InitializeComponent() call
      ' Create an AppDomain object
      Dim adCurrent As AppDomain = AppDomain.CurrentDomain
      ' Attach the UnhandledExceptionEventHanlder 
      ' to the UnhandledException of the AppDomain object
      AddHandler adCurrent.UnhandledException, _
       AddressOf UnhandledExceptionHandler

    End Sub

  6. Set the form as the startup object for the project.

  7. Run the project. Enter a zero for the number of gallons and click the button. An unhandled exception will occur, and a message box will be displayed notifying you that the exception has been logged. You can view the logged message by selecting Start, Control Panel, Administrative Tools, Event Viewer. The Event Viewer will display the Mileage Efficiency Calculator Log in the left pane of the window as shown in Figure 3.9. The right pane of the window shows the events that have been logged. You can double-click an event to view its description and other properties as shown in Figure 3.10.


Refreshing the List of Event Logs If you have the Event Viewer open already when you run this code, you'll need to close and reopen it to see the new event log category.

Figure 3.9 EventViewer showing the MileageEfficiencyCalculatorLog.

Figure 3.10 Event Properties of a particular event.

  • If the existing exception classes do not satisfy your exception handling requirements, you can create new exception classes specific to your application. Custom exceptions should be derived from the ApplicationException class.

  • You can use the UnhandledException event of the AppDomain class to manage unhandled exceptions.

  • You can use the EventLog class to log events to the Windows event log.

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