前言
在之前的文章中有講到AspNetCore多環境配置檔案的應用，我們根據自己多種環境分別配置多個appsettings.$EnvironmentName.json檔案。
在實際的開發中我們可能會遇到不只一個配置檔案，如當我們使用log4net日誌庫時，喜歡使用單獨的log4net.config配置檔案。並且我們還遇到不同環境下的配置檔案還存在差異。這時我們可能可以效仿appsettings.json多環境配置的風格實現多環境配置檔案。
配置log4net
新建Web專案
安裝Microsoft.Extensions.Logging.Log4Net.AspNetCore
建立兩個環境的配置檔案分別如圖，並設定檔案屬性使其在釋出時能夠複製到根目錄
兩個配置檔案內容如下，例如我們這裡生產環境的配置檔案多增加一個KafkaAppender使日誌傳送至kafka訊息丟列中，然後kafka的消費者將日誌消費至ES叢集,而本地開發的日誌則沒必要進行上傳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中新增log4net配置如下圖
// ===============多環境log4net配置===============
IWebHostEnvironment environment = builder.Environment;
var configName = "log4net" + (environment.IsProduction() ? string.Empty : environment.EnvironmentName) + ".config";
builder.Logging.AddLog4Net(configName, watch: true);
// ===============多環境log4net配置===============
現在所有的配置都完成了。