How to Properly Pull NLog DB ConnectionString from appsettings.json

Question

I have my nlog.config file setup with a database target and want to read connectionstring from appsettings.json (and for anyone interested, the value in appsettings is actually stored in Azure KeyVault). What is the proper way to do this? I have ASP.NET core 3.1 application.

nlog.config

<?xml version="1.0" encoding="utf-8" ?>
<!--The first nLog section internal log file logs the NLog configuration issues, Don't remove this-->
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true"
      internalLogLevel="debug"
      internalLogFile="c:\temp\foo.txt">


  <extensions>
    <add assembly="NLog.Extensions.Logging"/>
    <add assembly="NLog.Web.AspNetCore"/>
  </extensions>
  
    <variable name="defaultLayout" value="${date} ${threadid} ${uppercase:${level}} ${logger} ${ndc} - ${message}${newline}" />
    <variable name="logDirectory" value="./logs/${shortdate}"/>
    <variable name="apiFileLogPath" value="c:\temp\api-nlog.txt"/>
    
    <!-- the targets to write to -->
    <targets>
        <target name="dbTarget"
                xsi:type="Database"
                commandText="..."
            connectionString="${configsetting:item=NLogConfiguration.ConnectionString}">
            <!--<connectionString></connectionString>-->
        </target>
        <!-- write to the void aka just remove -->
        <target xsi:type="Null" name="blackhole" />
    </targets>

    <!-- rules to map from logger name to target -->
    <rules>

        <!--Skip Microsoft logs and so log only own logs-->
        <!--<logger name="*" minlevel="Trace" writeTo="blackhole" final="true" />-->
        <logger name="*" minlevel="Debug" writeTo="dbTarget" />
    </rules>
</nlog>

Relevant part of appsettings.json file:

{
  "NLogConfiguration": {
    "ConnectionString": "..."
  }
}

I have a breakpoint in my code to inspect the database target but the connectionString value doesn't get set. it is staying as ${configsetting:item=NLogConfiguration.ConnectionString}

I don't have any NLog code at startup. I have a wrapper class around NLog and am initializing it that way. That class is shown below:

DBLogManager

using System;
using System.Configuration;
using System.Globalization;
using System.Web;
using NLog;
using System.Xml;
using System.IO;
using System.Reflection;
using System.Text;
using Microsoft.Extensions.Configuration;

namespace DBLog.DBLogManager
{

    public class DBLogManager : IDBLogManager
    {
        static ILogger InitializeDBLogManager()
        {
            XmlDocument nLogConfig = new XmlDocument();
            var path = System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "nlog.config");

            NLog.LogManager.Configuration = new NLog.Config.XmlLoggingConfiguration(path);
            
            return LogManager.GetLogger("AppLogger");

        }
        private static readonly ILogger Logger = InitializeDBLogManager();
    }
}

Answer 1

For ${configsetting} you need at least the package NLog.Extensions.Logging or one of the dependent packages like NLog.Web.AspNetCore or NLog.Extensions.Hosting.

Normally you would use .UseNLog() on IHostBuilder, see NLog - Getting started with ASP.NET Core 3

To manual register the Microsoft Extension IConfiguration with ${configsetting}:

IConfigurationRoot config = new ConfigurationBuilder()
    .AddJsonFile(path: "AppSettings.json").Build();
NLog.Extensions.Logging.ConfigSettingLayoutRenderer.DefaultConfiguration = config;

See NLog - ConfigSetting Layout Renderer