Setting up log4net to write to trace.axd

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.

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