How does NHibernate implement optional log4net logging

log4net, nhibernate

Solution

NHibernate has an ILoggerFactory to create the logger defined as below:

public interface ILoggerFactory
    {
        IInternalLogger LoggerFor(string keyName);
        IInternalLogger LoggerFor(System.Type type);
    }

The `IInternalLogger` interface, that depites the name is not internal at all, is the actual logger interface, that non surprisingly map exactly the `ILog` interface of `log4net`. NH internally ask logger factory when it needs to log. If no other logger is configured, and log4net assembly is present, the factory return a wrapper around log4net. If you want provide your own you just need to provide a logger factory and configure it as:

 <appSettings>
        <add key="nhibernate-logger" 
                  value="MyLoggingLoggerFactory, MyLoggingAssembly"/>
    </appSettings>

As you guess, the class MyLoggingLoggerFactory must implement `ILoggerFactory`. The returned loggers of course must implement `IInternalLogger`, that as said before is public even if the unhappy name can be misleading. This is the startegy they use to be `log4net` independent. As an addition, if you don't want log at all, just remove the log4net assembly from your deploy directory: NH will degrade gracefully and use internally a null logger.

Problem

I remember the days (not too long ago, really) when log4net came along with NH and was compulsory for the OR-Mapper. Nowadays it isn't compulsory anymore, but optional. I really wonder how this is achieved by NHibernate. If anyone could explain in a few words, that would be highly appreciated!

Original source