以下配置了二种写文件，第一种根据日期写文件yyyyMMdd.txt，第二种是写固定文件login.txt。
1， 下载Log4net组件：
http://logging.apache.org/log4net/download.html
2，将log4net.dll加到项目引用中；
3，在app.config中添加相关配置信息，文件内容如下：
<?xml version="1.0"?>
<configuration>
<configSections>
<sectionGroup name="applicationSettings" type="System.Configuration.ApplicationSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
<section name="ChinaBank.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false"/>
</sectionGroup>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net-net-1.0"/>
</configSections>
<log4net>
<logger name="Daily.Logging">
<level value="INFO"/>
<appender-ref ref="DailyAppender" />
</logger>
<logger name="Login.Logging">
<level value="INFO"/>
<appender-ref ref="LoginAppender" />
</logger>
<appender name="DailyAppender" type="log4net.Appender.RollingFileAppender,log4net" >
<param name="File" value="Logs/" />
<param name="AppendToFile" value="true" />
<param name="RollingStyle" value="Date" />
<param name="DatePattern" value="yyyyMMdd.‘txt‘" />
<param name="StaticLogFileName" value="false" />
<layout type="log4net.Layout.PatternLayout,log4net">
<param name="ConversionPattern" value="%m%n" />
</layout>
</appender>
<appender name="LoginAppender" type="log4net.Appender.RollingFileAppender,log4net" >
<param name="File" value="Logs/login.txt" />
<param name="AppendToFile" value="true" />
<layout type="log4net.Layout.PatternLayout,log4net">
<param name="ConversionPattern" value="%d|%m|%n" />
</layout>
</appender>
</log4net>
<!--这里为appSettings等其它节点信息-->
</configuration>
4，在AssemblyInfo.cs最下面添加(其中XXXXX.exe.config为app.config在bin\Debug中生成的文件名，以实际为准)：
[assembly: log4net.Config.XmlConfigurator(ConfigFile = "./XXXXX.exe.config", ConfigFileExtension = "config", Watch = true)]
5，在项目中使用Log4net：
//根据日期写文件yyyyMMdd.txt
log4net.ILog log = log4net.LogManager.GetLogger("Daily.Logging");
log.Info("这是一条普通信息");
//写固定文件login.txt
log4net.ILog log2 = log4net.LogManager.GetLogger("Login.Logging");
log2.Info("用户张三登录");
相关引用：http://www.cnblogs.com/gdjlc/archive/2012/12/11/2812509.html