Use Multiple log4net Outputs from One Application

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Introduction

This is an article simply to demonstrate how to use several output log files depending on the library or the application.

Using the Code

Developers who develop class libraries may need to log details separate from the main application to support users or to quickly retrieve the data. But when they try to implement this, they will face a problem. This is because normally log4net has only one config section under the ASP.NET web.config file or Windows application app.config file. To overcome this problem, you can use different logger entries which define the output.

Default case:

<log4net>
    <appender type="log4net.Appender.RollingFileAppender" name="RollingFile">
        <file value="c:\\logs\\Log.txt" />
        <layout type="log4net.Layout.PatternLayout" />
            <conversionpattern value="%d [%t] %-5p %c - %m%n" />
        </layout>
    </appender>

    <root>
        <level value="ALL" />
        <appender-ref ref="RollingFile" />
    </root>
</log4net>

When you specify the <root /> tag, it says all the log data will log to that output file. To stop this, we need to remove the <root /> element and have separate <logger /> tags for every library or the application. You also need to specify your root namespace for the library.

<log4net>
    <appender type="log4net.Appender.RollingFileAppender" name="classApp1">
      <file value="c:\\Library1.txt" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionpattern value="%d [%t] %-5p %c - %m%n" />
      </layout>
    </appender>

    <appender type="log4net.Appender.RollingFileAppender" name="classApp2">
      <file value="c:\\Library2.txt" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionpattern value="%d [%t] %-5p %c - %m%n" />
      </layout>
    </appender>

    <appender type="log4net.Appender.RollingFileAppender" name="application">
      <file value="c:\\Application.txt" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionpattern value="%d [%t] %-5p %c - %m%n" />
      </layout>
    </appender>

    <logger name="ClassLibrary1">
      <level value="ERROR" />
      <maximumfilesize value="256KB" />
      <param value="ERROR" name="Threshold" />

      <appender-ref ref="classApp1" />
    </logger>

    <logger name="ClassLibrary2">
      <level value="WARN" />
      <maximumfilesize value="256KB" />
      <param value="WARN" name="Threshold" />

      <appender-ref ref="classApp2" />
    </logger>

    <logger name="WindowsApplication1">
      <level value="WARN" />
      <maximumfilesize value="256KB" />
      <param value="WARN" name="Threshold" />

      <appender-ref ref="application" />
    </logger>

  </log4net>

You can see three <logger /> items and separate tags referenced from those loggers. "name" of the <logger /> will specify the root namespace of the library or the application. Specify which appender you need to refer from the logger. When executing the application, if something is written to the log from the ClassLibrary2.Class2, it will refer the <logger name="ClassLibrary2" /> entry. This means the output will be on c:\\Library2.txt file. This way you can specify any amount of loggers dynamically.

Note: The code in Visual Studio 2005 format.