17.15 Write to the Windows Event Log

Problem

You need to write a log entry to the Windows event log on the current computer.

Solution

Register the event source and create the log if required. Write the message using the System.Diagnostics.EventLog class.

Discussion

The EventLog class makes it easy to write and retrieve event log entries. You create an EventLog instance that wraps a specific log—typically the all-purpose Application log or a custom log that's used by your application.

' Access the Application log. Dim Log As New EventLog("Application")

You can also supply an additional argument to the EventLog constructor to specify a computer name . This allows you to access an event log defined on another computer on the network.

' Access the Application log. Dim Log As New EventLog("Application", "ComputerName")

To retrieve log entries, use the EventLog.Entries property, which provides a collection of EventLogEntry instances. To write a log entry, call the EventLog.WriteEntry method, specifying a text message and optionally other information, including an event log type (error, warning information, or security audit). However, before you attempt to write a message, you should make sure the current application is registered to write in the log, and you should call the EventLog.CreateEventSource method if it's not.

Here's an example Console application that writes a single log entry into a custom log and the displays all the entries in the log:

Public Module EventLogTest Public Sub Main() ' Register the event source if needed. If Not EventLog.SourceExists("MyApp1") Then ' This registers the event source and creates the custom log, ' if needed. EventLog.CreateEventSource("MyApp1", "MyNewLog") End If ' Create a log instance. Dim Log As New EventLog("MyNewLog") Log.Source = "MyApp1" ' Write a message. Log.WriteEntry("This is a test message.", _ EventLogEntryType.Information) ' Display all messages. Dim Entry As EventLogEntry For Each Entry In Log.Entries Console.WriteLine("Message: " & Entry.Message) Console.WriteLine("Written By: " & Entry.Source) Console.WriteLine("Written At: " & Entry.TimeWritten.ToString()) Console.WriteLine("Type: " & Entry.EntryType.ToString()) Console.WriteLine() Next Console.ReadLine() End Sub End Module

This code will work only it if executes under an account that has permission to modify the event log. To grant this permission to an account that lacks it, you must use regedt32.exe and find the HKEY_Local_Machine\SYSTEM\CurrentControlSet\Services\EventLog key. Then right-click the EventLog key, and select Permissions. Any account that is granted Full Control of this folder will be able to write to the event log.