SELECT [softwareThoughts] from Tyler

In the Log4Net release notes there's a note about now to create your own custom pattern element in the logging configuration, so that, e.g., your conversion pattern might look like:

  "%-5level %logger - %myConverter - %message%newline"

Maybe someone can comment on why this would be better than what I will recommend. Anyways, if you take this approach and want to include information about the actual logging event, you should know two things:
1) subclassing PatternLayoutConverter instead of PatternConverter will give you a Convert method that includes a LoggingEvent object.

2) The state object passed to PatternConverter.Convert is actually a LoggingEvent object!

Great.
I found it easier to create a new appender, which simply added objects to the LoggingEvent's properties collection.

Here's the appender class:

public class PropertiesExpanderAppender : ForwardingAppender
{
  protected override void Append(LoggingEvent loggingEvent)
  {
      if (null != HttpContext.Current)
      {
          HttpContext context = HttpContext.Current;
          loggingEvent.Properties["USER_IP"] = context.Request.UserHostAddress;
          loggingEvent.Properties["REQUEST_URI"] = context.Request.Url.PathAndQuery;
      }

      if (null != UserProperties.CurrentUser)
      {
          UserProperties userProperties = UserProperties.CurrentUser;
          loggingEvent.Properties["USER_EMAIL"] = userProperties.UserName;
          loggingEvent.Properties["USER_NAME"] = userProperties.Name;
          loggingEvent.Properties["USER_ID"] = userProperties.UserId;
      }

      // Delegate to base class which will forward
      base.Append(loggingEvent);
  }
}

Here are the important snippets from the log4net configuration:

<appender name="PropertiesExpanderAppender" type="NameSpace.PropertiesExpanderAppender">
 <appender-ref ref="RollingLogFileAppender" />
</appender>
<appender name="RollingLogFileAppender" type="log4net.Appender.RollingFileAppender">
 ...
 <layout type="log4net.Layout.PatternLayout">
  <conversionPattern value="%date ... %P{USER_IP} %P{REQUEST_URI} %P{USER_EMAIL} %P{USER_NAME} ..." />
 </layout>
</appender>
<root>
 <level value="ALL" />
 <appender-ref ref="PropertiesExpanderAppender" />
</root>

Note the %P{USER_IP} pattern elements, which correspond to the properties added by the forwarding appender. Now you can add as many logging elements as you want, without having to add a bunch of classes.