Handling Different Types Of Events
Generally
, you log entries to the Windows 2000 Event Log when something significant happens in your
web application. These “events” fall into two categories: expected events and
unexpected events.
Expected Events
Expected events are things that are not completely out
of the ordinary, but you want to make a record of the fact that they did happen. Generally
, you determine whether an expected event happened by using a logic structure. Items can be
logged to the proper event log (or not logged at all), depending on the outcome of the
logic structure. Listings 8.5 and 8.6 provide examples.
Listing 8.5. Logging Expected Events with Logic
Structures (C#)
<%@ Page Language="C#" %>
<%@ Import Namespace="System.Diagnostics"%>
<script language="C#" runat="server">
//build a generic function for logging, to prevent redundant code
void LogStuff(string message, EventLogEntryType eventType,
short eventID)
{
EventLog el = new EventLog();
el.MachineName = "."; //local computer
el.Source = "Test Source";
el.Log = "Test Log";
if (!EventLog.SourceExists(el.Source))
{
//Event source doesn't exist, so create a new one
EventLog.CreateEventSource(el.Source,el.Log);
}
el.WriteEntry(message,eventType,eventID);
el.Close();
}
</script>
<%
//grab the "second" portion of the time
int second = DateTime.Now.Second;
//write to a different log, depending on the value
if(second % 2 == 0)
{
LogStuff("The value is even: " + second. ToString(),
EventLogEntryType.Information, 1);
}
else if(second == 7)
{
LogStuff("Beware of superstitions: " + second. ToString(),
EventLogEntryType.Warning, 2);
}
else
{
LogStuff("The value is odd: " + second. ToString(),
EventLogEntryType.Information, 3);
}
Response.Write("Done!");
%> |
Listing 8.6. Logging Expected Events with Logic Structures (Visual Basic .NET)
<%@ Page Language="VB" %>
<%@ Import Namespace="System.Diagnostics" %>
<script language="VB" runat="server">
'build a generic function for logging, to prevent redundant code
Sub LogStuff(message As String, eventType As EventLogEntryType, _
eventID as Short)
Dim el As EventLog = New EventLog()
el.MachineName = "." "local computer
el.Source = "Test Source"
el.Log = "Test Log"
If Not EventLog.SourceExists(el.Source) Then
‘Event source doesn't exist, so create a new one
EventLog.CreateEventSource(el.Source,el.Log)
End If
el.WriteEntry(message,eventType,eventID)
el.Close()
End Sub
</script>
<%
'grab the "second" portion of the time
Dim second As Integer = DateTime.Now.Second
'write to a different log, depending on the value
If second Mod 2 = 0 Then
LogStuff("The value is even: " & second. ToString(), _
EventLogEntryType.Information, 1)
ElseIf second = 7 Then
LogStuff("Beware of superstitions: " & second. ToString(), _
EventLogEntryType.Warning, 2)
Else
LogStuff("The value is odd: " & second. ToString(), _
EventLogEntryType.Information, 3)
End If
Response.Write("Done!")
%> |
The example starts by defining a LogStuff function. Because several different types of
log entries can be made on the page, it makes sense to consolidate the logic into a
utility function. In your own web applications, you might also want to encapsulate this
logic into a lightweight utility component. For now, a utility function will suffice.
The
example itself is not very complex. You retrieve the “second” portion of the current
time and write an entry to the custom event log based on the value that you obtain. Notice
that each call to LogStuff feeds in a different value for the event ID (the last
parameter). You can use the sorting capabilities of the Windows 2000 Event Log Viewer to
group similar events for analysis. Also, the EventLogEntryType parameter is set to Warning
when the number 7 (often considered lucky by superstitious people) comes up.
Unexpected
Events
Unexpected events happen when errors occur in your web application. Microsoft’s
.NET Framework provides structured error handling to capture and handle these errors.
Structured error handling also presents the perfect place to log these errors to the
Windows 2000 Event Log. The examples provided in Listings 8.7 and 8.8 will help clarify.
Listing 8.7. Logging Unexpected Events with Structured Error Handling (C#)
<%@ Page Language="C#" %>
<%@ Import Namespace="System.Diagnostics" %>
<script language="C#" runat="server">
//build a generic function for logging, to prevent redundant code
void LogStuff(string message, EventLogEntryType eventType,
short eventID)
{
//code truncated - see example 8.7
}
</script>
<%
int value1 = 10;
int value2 = 0;
int value3 = 0;
try
{
value3 = value1 / value2;
}
catch (DivideByZeroException e)
{
//log the error to the event log
LogStuff(e.Message, EventLogEntryType.Error, 1);
}
Response.Write("Done!");
%> |
Listing 8.8. Logging Unexpected Events with Structured Error Handling (Visual Basic .NET)
<%@ Page Language="VB" %>
<%@ Import Namespace="System.Diagnostics" %>
<script language="VB" runat="server">
'build a generic function for logging, to prevent redundant code
Sub LogStuff(message As String, eventType As EventLogEntryType, _
'code truncated - see example 8.8
End Sub
</script>
<%
Dim value1 As Integer = 10
Dim value2 As Integer = 0
Dim value3 As Integer = 0
Try
value3 = value1 / value2
Catch e As OverflowException
‘log the error to the event log
LogStuff(e.Message, EventLogEntryType.Error, 1)
End Try
Response.Write("Done!")
%> |
You start off by declaring three integer variables. Next, you open a structured error
handling “try” block. You then intentionally manufacture a DivideByZeroException
(OverflowException, in Visual Basic) by dividing value1 by value2 (which translates to 10 /
0). The “catch” block intercepts the exception because it was defined as the proper
type. Had the error not been of DivideByZeroException (OverflowException, in Visual Basic)
, a normal runtime error would have occurred on the page. To prevent this, you can declare
another catch block underneath it for the generic Exception class. That way, specific
exceptions will be caught, but if a strange exception occurs, then you’re still covered.