NLog-conditional-layout.config

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.nlog-project.org/schemas/NLog.xsd NLog.xsd"
      autoReload="true"
      throwExceptions="false"
      internalLogLevel="Off" internalLogFile="logs/nlog-internal.log">
  <!-- cf. https://github.com/NLog/NLog/wiki/Layout-Renderers -->
  <variable name="default_layout"
            value="${time} [${logger}(L\:${callsite-linenumber})] ${uppercase:${level}} (TID\:${threadid}) ${message}${onexception:${newline}${exception:format=ToString}}" />

  <!--
  	usage)
  	
    MappedDiagnosticsContext.Set("userid", UserId) // thread localized
  -->
  <variable name="default_layout"
            value="${time} [${logger}(L\:${callsite-linenumber})] ${uppercase:${level}} (TID\:${threadid}) ${message}${onexception:${newline}${exception:format=ToString}}" />

  <variable name="default_layout_with_user"
            value="${time} [${logger}(L\:${callsite-linenumber})] ${uppercase:${level}} (TID\:${threadid}, UID\:${mdlc:item=UserID}) ${message}${onexception:${newline}${exception:format=ToString}}" />

  <targets>
  	<!-- cf. https://github.com/NLog/NLog/wiki/Console-target -->
    <target xsi:type="Console"
            name="console"
            layout="${when:when=length('${mdlc:item=UserID}') > 0:inner=${default_layout_with_user}:else=${default_layout}}" />
  </targets>

  <rules>
    <logger name="*" minlevel="Debug" writeTo="console" />
  </rules>
</nlog>

NLog.config

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.nlog-project.org/schemas/NLog.xsd NLog.xsd"
      autoReload="true"
      throwExceptions="false"
      internalLogLevel="Off" internalLogFile="logs/nlog-internal.log">
  <!-- cf. https://github.com/NLog/NLog/wiki/Layout-Renderers -->
  <variable name="default_layout"
            value="${time} [${logger}(L\:${callsite-linenumber})] ${uppercase:${level}} (TID\:${threadid}) ${message}${onexception:${newline}${exception:format=ToString}}" />

  <targets>
  	<!-- cf. https://github.com/NLog/NLog/wiki/Console-target -->
    <target xsi:type="Console"
            name="console"
            layout="${default_layout}" />

	  <!-- cf. https://github.com/NLog/NLog/wiki/File-target -->
    <target xsi:type="File"
            name="file"
            layout="${default_layout}"
            fileName="logs/${shortdate}/MyApp.log"
            archiveFileName="logs/archive/${shortdate}/MyApp.log"
            archiveEvery="Day" 
            maxArchiveFiles="30"
            encoding="UTF-8" />

	  <!-- cf. https://github.com/NLog/NLog/wiki/Eventlog-target -->
    <target xsi:type="EventLog"
            name="eventlog"
            layout="${default_layout}"
            source="MyApp" />
  </targets>

  <rules>
    <logger name="*" minlevel="Debug" writeTo="console" />
    <logger name="*" minlevel="Debug" writeTo="file" />
    <logger name="*" minlevel="Info" writeTo="eventlog" />
  </rules>
</nlog>