A log4net Tutorial on .NET Application Logging

Logging frameworks are invaluable in application development. This log4net tutorial will look at it’s benefits as a logging framework, how to set it up and how to create entries.

The Benefits of Software Logging Include

  • Collecting data on the (mis)behavior of applications in production
  • Looking for errors in logic when using a debugger is altering the behavior of the program
  • Obtaining large amounts of behavioral data for subsequent analysis
  • Tracking user events – e.g. clickstreams
  • Monitoring errors and exceptions

Some logging frameworks, such as ELMAH and New Relic specialize in certain areas of logging. So, it’s worth considering your requirement carefully before selecting a framework. It’s possible that you may wish to use multiple frameworks.

We’ve all written our own logging frameworks at some point. After all, you can get pretty far with just writing to a text file. However, as in all development, it makes sense to use something that’s already been written—and, more importantly, tested—for you.

log4net is a popular general-purpose logging framework. It’s simple to use yet powerful. And, as it’s a well-known open-source project, it’s likely to be familiar to other developers and/or support staff who may need to use your logging system from time to time. At the very least they can find copious documentation and help online.

How to Set Up log4net

Installing and configuring log4net is one of the first things I do when writing a new application. It’s just handy to be able to log data at various stages of the development life cycle.

I’m going to give a brief introduction to setting up log4net within an application. There are numerous setup options, but we’ll go through a pretty common configuration.

Create a new console application and use NuGet to find and install the log4net package.

Open up AssemblyInfo.cs (under Properties in the Solution Explorer view of your application). Add the following line to the end of the file

[assembly: log4net.Config.XmlConfigurator(ConfigFile = "log4net.config", 
  Watch = true)]

This tells log4net that the configuration information (XML) is located in log4net.config.

Create a new log4net.config file in the root of the application and set its Copy to Output Directory to be Copy if newer. The new file should contain the following XML document

<?xml version="1.0"?>
  <appender name="RollingFile" type="log4net.Appender.RollingFileAppender">
    <file value="C:\Logs\Today.log" />
    <rollingStyle value="Composite" />
    <datePattern value="yyyyMMdd" />
    <appendToFile value="true "/>
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%level %logger %date{ISO8601} - %message%newline" />
    <maxSizeRollBackups value="1" />
    <maximumFileSize value="100MB" />
    <!-- Options are "ALL", "DEBUG", "INFO", "WARN", "ERROR", "FATAL" and "OFF". -->
    <level value="ALL" />
    <appender-ref ref="RollingFile" />

Details of the options are available online, but the highlights are

  • Log to C:\Logs\Today.log. Files will be renamed when they rollover, so Today.log is always the most recent.
  • Set maximum log file size to 100Mb
  • Use a rolling file logger—creates a new log file every day, or when the log file exceeds its maximum size
  • Set logging level to ALL—the most verbose setting
  • Format each log entry to contain the level, class doing the logging, date/time that the entry was added and a custom message

One of the joys of using a third-party logging system is all the features you get for nothing. Once log4net is configured, logging to a database, the console, the Windows event log, e-mail, a UDP connection, etc. is as simple as defining a new appender in the configuration file.

And, with that, we are ready to go. So, how do we actually create a new log entry?

How to Create Log Entries

In a class you want to log from add the following field

private static readonly ILog Log = log4net.LogManager

Then, from a method in the class, you can add a DEBUG entry using

this.Log.Debug("We are logging");

Want to add an INFO entry? Then it’s

this.Log.Info("We are logging");

There are also very useful formatting helpers that use the same syntax as string.Format

this.Log.DebugFormat("We are logging entry #{0}", 3);

The resulting entry in C:\Logs\Today.log will look like

DEBUG ConsoleApplication1.Program 2015-07-17 13:27:03,079 - We are logging entry #3

And that’s all there is to it. Well, actually, it’s far from all there is to it—there’s a lot more to log4net—but you should now be up and running.

Need to Learn More About .NET?

I hope this log4net tutorial has been helpful.  If you want to learn more about .NET development, Learning Tree has a comprehensive range of over 20 .NET courses covering all the major topics.  Many of these are 1-day online sessions you can take from the convenience of home or office.

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