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Writing Entries to Event Logs

When you write an entry to an event log, you specify the message you want to write to the log as a string. A message should contain all information needed to interpret what caused the problem and what to do to correct the problem.

There are two ways you can write an entry to the log; both are equally valid. The most direct way is to register an event source with the log to which you want to write, then instantiate a component and set its Source property to that log, and finally call WriteEntry. If you do this, you do not have to set the Log property for the component instance; the log is automatically determined when you connect to a source that has already been registered. For more information on registering a source, see Adding Your Application as a Source of Event Log Entries.

The other way to approach this process is to instantiate an EventLog component, set its Source, Machine, and Log properties, and call the WriteEntry method. In this case, the WriteEntry method would determine whether the source already existed and register it on the fly if it did not.

The following conditions must be met in order to successfully write a log entry:

  • The source must be registered with the desired log.
    Note   You must set the Source property on your EventLog component instance before you can write entries to a log. When your component writes an entry, the system automatically checks to see if the source you specified is registered with the event log to which the component is writing, and calls CreateEventSource if needed.
  • The message you specify cannot be more than 16K in length.
  • Your application must have write access to the log to which it is attempting to write. For more information, see Security Ramifications of Event Logs.

You can specify several parameters when you write an entry, including the type of entry you're making, an ID that identifies the event, a category, and any binary data you want to append to the entry. For more information on the properties associated with an entry, see EventLog Members.

To write an event log entry

  1. Instantiate an EventLog component. For more information, see Creating EventLog Component Instances.
  2. Use the EventLog.CreateEventSource method to register an event source with the log to which you want to write an entry, using a unique string for the source string. Set the Source property for the component to the source you registered. For more information, see Configuring EventLog Component Instances. Call the WriteEntry method to specify the entry to be written to the log.
    ' Visual Basic
    If Not EventLog.SourceExists("MyApp1") Then
        EventLog.CreateEventSource("MyApp1", "Application")
    End If
    EventLog1.Source = "MyApp1"
    EventLog1.WriteEntry("This is a simple event log entry")
    // C#
    if (!System.Diagnostics.EventLog.SourceExists("MyApp1"))
          "MyApp1", "Application");
    EventLog1.Source = "MyApp";
    EventLog1.WriteEntry ("This is a simple event log entry");

See Also

Introduction to the EventLog Component | Configuring EventLog Component Instances | Creating EventLog Component Instances | EventLog Members | Adding Your Application as a Source of Event Log Entries | Walkthrough: Exploring Event Logs, Event Sources, and Entries