Log4Net Config

Global.asax.cs

protected void Application_Start()
{
    log4net.Config.XmlConfigurator.Configure();
    log4net.GlobalContext.Properties["userid"] = new Log4NetUserIdResolver();
}

Log4NetUserIdResolver.cs

internal sealed class Log4NetUserIdResolver
{
    public override string ToString()
    {
        var userId = (HttpContext.Current?.User as WhateverPrincipal)?.AccountId;
        return userId ?? "none";
    }
}

web.config

  <!-- all the way at the top, just inside the configuration node. The section has to be defined before the parser 
       encounters the log4net node -->
  <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
  </configSections
  
  <!-- this can be somewhere down below appSettings -->
  <log4net>
    <root>
      <level value="ALL" />
      <appender-ref ref="RollingLogFileAppender" />
    </root>
    <!-- logs to App_Data/logs/log.txt, but stores a copy and restarts the file every day -->
    <appender name="RollingLogFileAppender" type="log4net.Appender.RollingFileAppender">
      <file value=".\\App_Data\\logs\\log.txt" />
      <appendToFile value="true" />
      <rollingStyle value="Date" />
      <datePattern value=".yyyyMMdd.txt" />
      <lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
      <layout type="log4net.Layout.PatternLayout">
        <!-- line layout includes the property{userid} to get from GlobalContext. See Global.asax.cs -->
        <conversionPattern value="%date %-5level &lt;%-7property{userid}&gt; %-30logger - %message%newline" />
      </layout>
    </appender>
  </log4net>