I’ve just been looking at logging info from MVC sites through to ASP.NET’s trace.axd handler. Here’s what you need to do.

1) Install log4net from NuGet;

Install-Package log4net

2) Tell log4net to read its config from XML – specifically, from your web.config file;

// Global.asax.cs;
 protected void Application_Start()
 {
   ...
   XmlConfigurator.Configure();
   ...
 }

3) Update web.config with three sections – one to register the log4net element, the log4net config itself, and the ‘trace’ element to turn on asp.net tracing;

web.config, part 1 — register the config section;

<configuration>
  <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />

web.config, part 2 — add the log4net element under <configuration>;

<log4net debug="true">
  <!-- writes to normal .net trace, eg DBGVIEW.exe, VS Output window -->
  <appender name="TraceAppender" type="log4net.Appender.TraceAppender">
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level - %message%newline" />
    </layout>
  </appender>
 
  <!-- writes to ASP.NET Tracing, eg Trace.axd -->
  <appender name="AspNetTraceAppender" type="log4net.Appender.AspNetTraceAppender" >
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger [%property{NDC}] - %message%newline" />
    </layout>
  </appender>
 
  <!-- writes errors to the event log -->
  <appender name="EventLogAppender" type="log4net.Appender.EventLogAppender">
    <applicationName value="AI Track Record" />
    <threshold value="ERROR" />
    <layout type="log4net.Layout.PatternLayout">
     <conversionPattern value="%date [%thread] %-5level %logger [%property{NDC}] - %message%newline" />
    </layout>
  </appender>
 
  <root>
    <level value="ALL" />
    <appender-ref ref="AspNetTraceAppender" />
    <appender-ref ref="EventLogAppender" />
    <appender-ref ref="TraceAppender" />
  </root>
 
</log4net>

web.config, part 3 – enable trace element in <configuration> / <system.web>

 
<configuration>
  <system.web>
    <trace writeToDiagnosticsTrace="true" enabled="true" pageOutput="false" requestLimit="250" mostRecent="true" />
  <system.web>
</configuration>

4) Finally, to actually use the log, you need to create a logger;

// maybe add this to each class, or to be fancy, inject using Ninject;
private static readonly ILog log = LogManager.GetLogger(typeof(MyClassName));

and write to it using methods like Info, Debug, etc;

log.Debug("Here's my excellent message");

5) Check Trace.axd! You’ll see your messages written to the ‘Trace Information’ section

Summary

Things to note — each appender has its own format, in the <converstionPattern> element, and its own ‘seriousness’, in the <threshold> element. So you can set up fine-grained control, like logging absolutely everything to a file and only errors to event log, or writing sparse information to trace.axd while writing verbose messages to a database table.