前言
在之前的文章中有讲到AspNetCore多环境配置文件的应用，我们根据自己多种环境分别配置多个appsettings.$EnvironmentName.json文件。
在实际的开发中我们可能会遇到不只一个配置文件，如当我们使用log4net日志库时，喜欢使用单独的log4net.config配置文件。并且我们还遇到不同环境下的配置文件还存在差异。这时我们可能可以效仿appsettings.json多环境配置的风格实现多环境配置文件。
配置log4net
新建Web项目
安装Microsoft.Extensions.Logging.Log4Net.AspNetCore
创建两个环境的配置文件分别如图，并设置文件属性使其在发布时能够复制到根目录
两个配置文件内容如下，例如我们这里生产环境的配置文件多增加一个KafkaAppender使日志发送至kafka消息丢列中，然后kafka的消费者将日志消费至ES集群,而本地开发的日志则没必要进行上传ES。
<!--log4net.config-->
<?xml version="1.0" encoding="utf-8" ?>
<log4net>
<!-- If you are looking here and want more output, first thing to do is change root/priority/@value to "INFO" or "ALL". -->
<root>
Value of priority may be ALL, DEBUG, INFO, WARN, ERROR, FATAL, OFF.
<priority value="ALL" />
<appender-ref ref="error-file" />
<appender-ref ref="debug-file" />
<appender-ref ref="KafkaAppender" />
</root>
<!-- Example of turning on the output from a component or namespace. -->
<logger name="Common">
<appender-ref ref="debugger"/>
<priority value="DEBUG" />
</logger>
<appender name="KafkaAppender" type="log4net.Kafka.Appender.KafkaAppender, log4net.Kafka.Appender">
<KafkaSettings>
<brokers>
<add value="127.0.0.1:9092" />
</brokers>
<topic type="log4net.Layout.PatternLayout">
<conversionPattern value="kafka.logstash" />
</topic>
</KafkaSettings>
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %level% [%t] %logger - %message" />
</layout>
</appender>
<appender name="debugger" type="log4net.Appender.DebugAppender">
<!-- Sends log messages to Visual Studio if attached. -->
<immediateFlush value="true" />
<layout type="log4net.Layout.SimpleLayout" />
</appender>
<appender name="debug-file" type="log4net.Appender.RollingFileAppender">
<param name="Encoding" value="utf-8" />
<file value="Logs/debug" />
<appendToFile value="true" />
<!-- Immediate flush on error log, to avoid data loss with sudden termination. -->
<immediateFlush value="true" />
<staticLogFileName value="false" />
<rollingStyle value="Date" />
<datepattern value="-yyyy.MM.dd'.log'" />
<!-- Prevents Orchard.exe from displaying locking debug messages. -->
<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %level% [%property{trace}] %logger - %message%newline" />
</layout>
</appender>
<appender name="error-file" type="log4net.Appender.RollingFileAppender">
<param name="Encoding" value="utf-8" />
<file value="Logs/error" />
<appendToFile value="true" />
<!-- Immediate flush on error log, to avoid data loss with sudden termination. -->
<immediateFlush value="true" />
<staticLogFileName value="false" />
<rollingStyle value="Date" />
<datepattern value="-yyyy.MM.dd'.log'" />
<!-- Prevents Orchard.exe from displaying locking debug messages. -->
<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
<filter type="log4net.Filter.LevelRangeFilter">
<!-- Only ERROR and FATAL log messages end up in this target, even if child loggers accept lower priority. -->
<levelMin value="ERROR" />
</filter>
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date [%t] %logger - %message [%P{Url}]%newline" />
</layout>
</appender>
</log4net>
<!--log4net.Development.config-->
<?xml version="1.0" encoding="utf-8" ?>
<log4net>
<!-- If you are looking here and want more output, first thing to do is change root/priority/@value to "INFO" or "ALL". -->
<root>
Value of priority may be ALL, DEBUG, INFO, WARN, ERROR, FATAL, OFF.
<priority value="ALL" />
<appender-ref ref="error-file" />
<appender-ref ref="debug-file" />
</root>
<!-- Example of turning on the output from a component or namespace. -->
<logger name="Common">
<appender-ref ref="debugger"/>
<priority value="DEBUG" />
</logger>
<appender name="debugger" type="log4net.Appender.DebugAppender">
<!-- Sends log messages to Visual Studio if attached. -->
<immediateFlush value="true" />
<layout type="log4net.Layout.SimpleLayout" />
</appender>
<appender name="debug-file" type="log4net.Appender.RollingFileAppender">
<param name="Encoding" value="utf-8" />
<file value="Logs/debug" />
<appendToFile value="true" />
<!-- Immediate flush on error log, to avoid data loss with sudden termination. -->
<immediateFlush value="true" />
<staticLogFileName value="false" />
<rollingStyle value="Date" />
<datepattern value="-yyyy.MM.dd'.log'" />
<!-- Prevents Orchard.exe from displaying locking debug messages. -->
<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %level% [%property{trace}] %logger - %message%newline" />
</layout>
</appender>
<appender name="error-file" type="log4net.Appender.RollingFileAppender">
<param name="Encoding" value="utf-8" />
<file value="Logs/error" />
<appendToFile value="true" />
<!-- Immediate flush on error log, to avoid data loss with sudden termination. -->
<immediateFlush value="true" />
<staticLogFileName value="false" />
<rollingStyle value="Date" />
<datepattern value="-yyyy.MM.dd'.log'" />
<!-- Prevents Orchard.exe from displaying locking debug messages. -->
<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
<filter type="log4net.Filter.LevelRangeFilter">
<!-- Only ERROR and FATAL log messages end up in this target, even if child loggers accept lower priority. -->
<levelMin value="ERROR" />
</filter>
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date [%t] %logger - %message [%P{Url}]%newline" />
</layout>
</appender>
</log4net>
在Program.cs中添加log4net配置如下图
// ===============多环境log4net配置===============
IWebHostEnvironment environment = builder.Environment;
var configName = "log4net" + (environment.IsProduction() ? string.Empty : "." + environment.EnvironmentName) + ".config";
builder.Logging.AddLog4Net(configName, watch: true);
// ===============多环境log4net配置===============
现在所有的配置都完成了。
__EOF__
