log4net enhancement for adding exception into to log4net, especially EventLog based appender. View http://blog.scooletz.com/2013/04/15/enhancing-log4net-exception-logging for more information

ExceptionBasedLogEnhancer.cs

using System;
using log4net.Core;
using log4net.Filter;
using log4net.Util;

namespace Log4NetEnhancing
{
    public class ExceptionBasedLogEnhancer : FilterSkeleton
    {
        private const string EventLogKeyEventId = "EventID";
        private const string EventLogKeyCategory = "Category";

        public override FilterDecision Decide(LoggingEvent loggingEvent)
        {
            var ex = loggingEvent.ExceptionObject;
            if (ex != null)
            {
                EnhanceWithException(loggingEvent.Properties, ex);
            }

            return FilterDecision.Neutral;
        }

        private static void EnhanceWithException(PropertiesDictionary properties, Exception ex)
        {
            if (ex.Source != null)
            {
                properties["ExceptionSource"] = ex.Source;

                // for event logs
                properties[EventLogKeyEventId] = GetEventId(ex);
                properties[EventLogKeyCategory] = GetCategory(ex);
            }
        }

        private static int GetEventId(Exception ex)
        {
            // more fancy implementation, like getting hash of ex properties can be provided, or mapping types of exceptions to eventids
            // return no more than short.MaxValue, otherwise the EventLog will throw
            
            return 0;
        }

        private static short GetCategory(Exception ex)
        {
            // more fancy implementation, like getting hash of ex properties can be provided, or mapping types of exceptions to category
            return 0;
        }
    }
}

log4net.config.xml

<appender name="MyEventLogAppender" type="log4net.Appender.EventLogAppender">
      <filter type="Log4NetEnhancing.ExceptionBasedLogEnhancer, Log4NetEnhancing"></filter>
</appender>