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log4net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。