How can I configure NLog in .NET Core with appsettings.json instead of an nlog.config file?

Question

The NLog documentation explains how to configure NLog for .NET Core applications by using an nlog.config XML file. However, I'd prefer to have just one configuration file for my application - appsettings.json. For .NET Framework apps, it's possible to put the NLog configuration in app.config or web.config. Is it possible to put the NLog config in appsettings.json in the same way?

For example, how could I put this configuration example from the NLog documentation for ASP.NET Core 2 into appsettings.json?

<?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"
      internalLogLevel="Info"
      internalLogFile="c:\temp\internal-nlog.txt">

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

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

    <!-- another file log, only own logs. Uses some ASP.NET core renderers -->
    <target xsi:type="File" name="ownFile-web" fileName="c:\temp\nlog-own-${shortdate}.log"
            layout="${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}|url: ${aspnet-request-url}|action: ${aspnet-mvc-action}" />
  </targets>

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

    <!--Skip non-critical Microsoft logs and so log only own logs-->
    <logger name="Microsoft.*" maxlevel="Info" final="true" /> <!-- BlackHole without writeTo -->
    <logger name="*" minlevel="Trace" writeTo="ownFile-web" />
  </rules>
</nlog>

Answer 1

Yes, this is possible but has a minimum version requirement. You must be using NLog.Extensions.Logging >= 1.5.0. Note that for ASP.NET Core applications this will be installed as a dependency if you install NLog.Web.AspNetCore >= 4.8.2.

You can then create an NLog section in appsettings.json and load it with the following code:

var config = new ConfigurationBuilder()
  .SetBasePath(System.IO.Directory.GetCurrentDirectory())
  .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true).Build();
NLog.Config.LoggingConfiguration nlogConfig = new NLogLoggingConfiguration(config.GetSection("NLog"));

For example, for an ASP.NET Core application, your Main() method in Program.cs should look something like this:

public static void Main(string[] args)
{
    var config = new ConfigurationBuilder()
        .SetBasePath(System.IO.Directory.GetCurrentDirectory())
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true).Build();
    LogManager.Configuration = new NLogLoggingConfiguration(config.GetSection("NLog"));

    var logger = NLog.Web.NLogBuilder.ConfigureNLog(LogManager.Configuration).GetCurrentClassLogger();
    try
    {
        logger.Debug("Init main");
        CreateWebHostBuilder(args).Build().Run();
    }
    catch (Exception ex)
    {
        logger.Error(ex, "Stopped program because of exception");
    }
    finally {
        LogManager.Shutdown();
    }
}

A configuration like the one in the question can be achieved with the following settings in appsettings.json:

"NLog":{
    "internalLogLevel":"Info",
    "internalLogFile":"c:\\temp\\internal-nlog.txt",
    "extensions": [
      { "assembly": "NLog.Extensions.Logging" },
      { "assembly": "NLog.Web.AspNetCore" }
    ],
    "targets":{
        "allfile":{
            "type":"File",
            "fileName":"c:\\temp\\nlog-all-${shortdate}.log",
            "layout":"${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}"
        },
        "ownFile-web":{
            "type":"File",
            "fileName":"c:\\temp\\nlog-own-${shortdate}.log",
            "layout":"${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}|url: ${aspnet-request-url}|action: ${aspnet-mvc-action}"
        }
    },
    "rules":[
        {
            "logger":"*",
            "minLevel":"Trace",
            "writeTo":"allfile"
        },
        {
            "logger":"Microsoft.*",
            "maxLevel":"Info",
            "final":"true"
        },
        {
            "logger":"*",
            "minLevel":"Trace",
            "writeTo":"ownFile-web"
        }
    ]
}

Edit: Thanks to Rolf Kristensen (who developed this functionality for NLog in the first place!) for pointing out this wiki page with more documentation on this feature: https://github.com/NLog/NLog.Extensions.Logging/wiki/NLog-configuration-with-appsettings.json