Why do loggers recommend using a logger per class?

Question

As per NLog's documentation:

Most applications will use one logger per class, where the name of the logger is the same as the name of the class.

This is the same way that log4net operates. Why is this a good practice?

Answer 1

With log4net, using one logger per class makes it easy to capture the source of the log message (ie. the class writing to the log). If you don't have one logger per class, but instead have one logger for the entire app, you need to resort to more reflection tricks to know where the log messages are coming from.

Compare the following:

Log per class

using System.Reflection;
private static readonly ILog _logger = 
    LogManager.GetLogger(MethodBase.GetCurrentMethod().DeclaringType);    

public void SomeMethod()
{
    _logger.DebugFormat("File not found: {0}", _filename);
}

One logger per app (or similar)

Logger.DebugFormat("File not found: {0}", _filename); // Logger determines caller

-- or --

Logger.DebugFormat(this, "File not found: {0}", _filename); // Pass in the caller

Using the second example, the Logger would need to build a stack trace to see who was calling it or your code would always have to pass in the caller. With the logger-per-class style, you still do this, but you can do it once per class instead of once per call and eliminate a serious performance problem.