When Log4Net doesn’t work, it can be a very frustrating experience.  Unlike your typical application library, log4net doesn’t throw exceptions when it fails.  Well that is to be expected and makes a lot of sense since it is a logging library.  I wouldn’t want my application to fail because it had trouble logging a message.

Unfortunately, the downside of this is that problems with log4net aren’t immediately apparent.  99.9% of the time, when Log4Net doesn’t work, it is a configuration issue.  Here are a couple of troubleshooting tips that have helped me out.

Enable Internal Debugging

This tip is straight from the Log4Net FAQ, but not everyone notices it. To enable internal debugging, add the following app setting to your App.config (or Web.config for web applications) file.

<add key="log4net.Internal.Debug" value="true"/>

This will write internal log4net messages to the console as well as the System.Diagnostics.Trace system.  You can easily output the log4net internal debug messages by adding a trace listener.  The following snippet is taken from the log4net FAQ and goes in your <configuration> section of your application config file.

<system.diagnostics>
  <trace autoflush="true">
    <listeners>
      <add 
        name="textWriterTraceListener" 
        type="System.Diagnostics.TextWriterTraceListener" 
        initializeData="C:\tmp\log4net.txt" />
    </listeners>
  </trace>
</system.diagnostics>

Passing Nulls For Value Types Into AdoNetAppender {.clear}

Another common problem I’ve dealt with is logging using the AdoNetAppender. In particular, attempting to log a null value into an int parameter (or other value type), assuming your stored procedure allows null for that parameter.

The key here is to use the RawPropertyLayout for that parameter. Here is a snippet from a log4net.config file that does this.

<parameter>
  <parameterName value="@BlogId" />
  <dbType value="Int32" />
  <layout type="log4net.Layout.RawPropertyLayout">
    <key value="BlogId" />
  </layout>
</parameter>

Hopefully this helps you with your log4net issues.