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。