With Log4net, you can configure the layout of the string that you want to log.
It's very simple, because you can change the pattern layout string in few seconds and without compiling the application.
You don't compile everytime change the pattern layout string, because it is in the application config.
For web application it is in the web.config file and for desktop application it is in the app.config.
Each pattern member starts with % and is followeb by the name of pattern member.
You can change the width, padding, left and right justification for each pattern member.

Sometimes, I use this layout pattern: %type %file %line %method %location %class %C %F %L %l %M %n , but below I explain every single member in order to you can create your pattern layout.

a or appdomain Friendly name of appdomain
c or logger Used to output the logger of the logging event
C or class or type The fully type name of the caller class
d or date The date of logging event in the local time zone
exception The exception
F or file The file name where the logging request was issued
identity or u The active user
l or location Location information of the caller
L or line The line number from where the logging request was issued
level or p The level of the logging event
m or message Application supplied message associated with the logging event
M or method The method name where the logging request was issued
n or newline Line separator
r or timestamp The timestamp when the logging request was issued
t or thread The name of thread that generated the logging event
username or w The Windows Identity for active user

In the previous article on the Log4net configuration, I explain how to configure Log4net with the file appender.
In this article I explain how to configure Log4net with the Sql Server appender.
It is very similar, but we see in detail the new configuration.

Download

You can download the latest version of Log4net from this location.

Add reference to your solution

In Visual Studio 2005 select Project -> Add Reference.
In the tab Browse, find and select the dll Log4net.dll in your local folder.

Modify web.config

Into web.config file add this code into the section Configuration->Configsections:

<configSections>
    <section name="log4net"
      type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" /> 
</configSections>

<log4net>
  <appender name="ADONetAppender" type="log4net.Appender.ADONetAppender">
    <bufferSize value="100" />
    <connectionType value="System.Data.SqlClient.SqlConnection, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
    <connectionString value="server=localhost; uid=; pwd=; database=" />
    <commandText value="INSERT INTO Log ([Date],[Thread],[Level],[Logger],[Message],[Exception]) VALUES
       (@log_date, @thread, @log_level, @logger, @message, @exception)" />
    <parameter>
      <parameterName value="@log_date" />
      <dbType value="DateTime" />
      <layout type="log4net.Layout.RawTimeStampLayout" />
    </parameter>
    <parameter>
      <parameterName value="@thread" />
      <dbType value="String" />
      <size value="32" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%t" />
      </layout>
    </parameter>
    <parameter>
      <parameterName value="@log_level" />
      <dbType value="String" />
      <size value="512" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%p" />
      </layout>
    </parameter>
    <parameter>
      <parameterName value="@logger" />
      <dbType value="String" />
      <size value="512" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%c" />
      </layout>
    </parameter>
    <parameter>
      <parameterName value="@message" />
      <dbType value="String" />
      <size value="4000" />
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%m" />
      </layout>
    </parameter>
    <parameter>
      <parameterName value="@exception" />
      <dbType value="String" />
      <size value="2000" />
      <layout type="log4net.Layout.ExceptionLayout" />
    </parameter>
  </appender>
  <root>
    <level value="DEBUG" />
    <appender-ref ref="ADONetAppender" />
  </root>
</log4net>

Change the connectionstring parameters to connect to your database.

Create the table

In your database, create the table to use Log4net. 

CREATE TABLE [dbo].[Log](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [Date] [datetime] NULL,
    [Thread] [varchar](255) NULL,
    [Level] [varchar](50) NULL,
    [Logger] [varchar](255) NULL,
    [Message] [varchar](4000) NULL,
    [Exception] [varchar](2000) NULL
) ON [PRIMARY]

Edit the Global.asax file

In the event "Application_Start" of the file Global.asax add this line:

log4net.Config.XmlConfigurator.Configure();

Begin to log

In every page you want to log something, add the static variable like below:
private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

This class has 5 levels of severity to log the operations:

log.Debug("log something at this level")
log.Info("log something at this level")
log.Warn("log something at this level");
log.Error("log something at this level");
log.Fatal("log something at this level");

You find the official documentation at this link.

If you want to log on the file system, read the previous article on Log4net.

Introduction

Log the actions of your applications is very important especially when you develop new features or develop very difficult logical business.
But it is also important when users use your applications to understand the critical issues or problems.
To implement quickly the log operations Apache developed an opensource library for .Net developers.

Download

You can download the latest version of Log4net from this location.

Add reference to your solution

In Visual Studio 2005/2008 select Project -> Add Reference.
In the tab Browse, find and select the dll Log4net.dll in your local folder.

Modify web.config

Into web.config file add this code into the section Configuration->Configsections:

<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net"/>

Yet, add the section:

<log4net>

<appender name="RollingFile" type="log4net.Appender.RollingFileAppender">

<file value="c:\temp\web.log" />

<appendToFile value="true" />

<maximumFileSize value="1024KB" />

<maxSizeRollBackups value="10" />

<layout type="log4net.Layout.PatternLayout">

<conversionPattern value="%date %level %logger - %message%newline" />

</layout>

</appender>

<root>

<level value="DEBUG" />

<appender-ref ref="RollingFile" />

</root>

</log4net>

 In this configuration will create a log file into the folder "c:\temp\" until its size is 1024Kb.
After this size the name of file will be web.log.1 until 10.
At this link you can find another configurations.

Global.asax

In the event "Application_Start" of the file Global.asax add this line:

log4net.Config.XmlConfigurator.Configure();

Begin to log

In every page you want to log something, add the static variable like below:

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

This class has 5 levels of severity to log the operations:

log.Debug("log something at this level")
log.Info("log something at this level")
log.Warn("log something at this level");
log.Error("log something at this level");
log.Fatal("log something at this level");

Simple and fast to use.

You find the official documentation at this link.