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

Two reasons immediately spring to mind:

1. Having a separate log for each class makes it easy to group together all log messages/errors pertaining to a given class.
2. Having a log within a class allows you to log internal details which may not be accessible outside the class (e.g., private state, information dealing with a class's implementation, etc.).

Answer 2

Probably because you want to be able to log methods that are only visible to the class without breaking encapsulation, this also makes it easy to use the class in another application without breaking the logging functionality.

Answer 3

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.

Answer 4

From a development standpoint, it's easiest if you don't have to create a logger object each time. On the other hand, if you don't, but rather you create it dynamically using reflection, it'll slow down performance. To solve this, you can use the following code which creates the logger dynamically asynchronously:

using NLog;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WinForms
{
    class log
    {

        public static async void Log(int severity, string message)
        {
            await Task.Run(() => LogIt(severity, message));
        }

        private static void LogIt(int severity, string message)
        {
            StackTrace st = new StackTrace();
            StackFrame x = st.GetFrame(2);     //the third one goes back to the original caller
            Type t = x.GetMethod().DeclaringType;
            Logger theLogger = LogManager.GetLogger(t.FullName);

            //https://github.com/NLog/NLog/wiki/Log-levels
            string[] levels = { "Off", "Trace", "Debug", "Info", "Warn", "Error", "Fatal" };
            int level = Math.Min(levels.Length, severity);
            theLogger.Log(LogLevel.FromOrdinal(level), message);

        }
    }
}