1 配置

1.1 安装依赖

使用nuget安装依赖，只需要安装NLog.Web.AspNetCore即可，NLog.Web.AspNetCore会自动依赖NLog。

1.2 依赖注入

修改Program.cs文件

1.3 添加nlog配置文件

添加nlog.config配置文件，添加在项目的根目录，配置文件内容在github拷贝即可。github地址：https://github.com/NLog/NLog/wiki/Getting-started-with-ASP.NET-Core-3
配置文件内容：

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true"
      throwConfigExceptions="true"
      internalLogLevel="info"
      internalLogFile="c:\temp\internal-nlog-AspNetCore3.txt">

  <!-- enable asp.net core layout renderers -->
  <extensions>
    <add assembly="NLog.Web.AspNetCore"/>
  </extensions>

  <!-- the targets to write to -->
  <targets>
    <!-- File Target for all log messages with basic details -->
    <target xsi:type="File" name="allfile" fileName="c:\temp\nlog-AspNetCore3-all-${shortdate}.log"
            layout="${longdate}|${event-properties:item=EventId_Id:whenEmpty=0}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}" />

    <!-- File Target for own log messages with extra web details using some ASP.NET core renderers -->
    <target xsi:type="File" name="ownFile-web" fileName="c:\temp\nlog-AspNetCore3-own-${shortdate}.log"
            layout="${longdate}|${event-properties:item=EventId_Id:whenEmpty=0}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}|url: ${aspnet-request-url}|action: ${aspnet-mvc-action}|" />

    <!--Console Target for hosting lifetime messages to improve Docker / Visual Studio startup detection -->
    <target xsi:type="Console" name="lifetimeConsole" layout="${level:truncate=4:lowercase=true}: ${logger}[0]${newline}      ${message}${exception:format=tostring}" />
  </targets>

  <!-- rules to map from logger name to target -->
  <rules>
    <!--All logs, including from Microsoft-->
    <logger name="*" minlevel="Trace" writeTo="allfile" />

    <!--Output hosting lifetime messages to console target for faster startup detection -->
    <logger name="Microsoft.Hosting.Lifetime" minlevel="Info" writeTo="lifetimeConsole, ownFile-web" final="true" />

    <!--Skip non-critical Microsoft logs and so log only own logs-->
    <logger name="Microsoft.*" maxlevel="Info" final="true" />  <!-- BlackHole -->

    <logger name="*" minlevel="Trace" writeTo="ownFile-web" />
  </rules>
</nlog>

2 常见问题

2.1 使用变量指定日志文件夹

<variable name="logdir" value="Log/${shortdate}"></variable>
  
<!-- the targets to write to -->
<targets>
  <!-- File Target for all log messages with basic details -->
  <target xsi:type="File" name="allfile" fileName="${logdir}/all.log"
          layout="${longdate}|${event-properties:item=EventId_Id:whenEmpty=0}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}" />

  <!-- File Target for own log messages with extra web details using some ASP.NET core renderers -->
  <target xsi:type="File" name="ownFile-web" fileName="${logdir}/own-web.log"
          layout="${longdate}|${event-properties:item=EventId_Id:whenEmpty=0}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}|url: ${aspnet-request-url}|action: ${aspnet-mvc-action}|" />

  <!--Console Target for hosting lifetime messages to improve Docker / Visual Studio startup detection -->
  <target xsi:type="Console" name="lifetimeConsole" layout="${level:truncate=4:lowercase=true}: ${logger}[0]${newline}      ${message}${exception:format=tostring}" />
</targets>

2.2 不输出库信息，只记录我们自己输出的信息

<rules>
  <logger name="Microsoft.*" maxlevel="Info" final="true" />
  <logger name="*" minlevel="Trace" writeTo="ownFile-web" />
</rules>