This article shows how the NLog connection string for the DatabaseTarget can be configured in the appsettings.json in an ASP.NET Core project and not the XML nlog.config file. All the NLog target properties can be configured in code if required and not just in the NLog XML configuration file.

Code: https://github.com/damienbod/AspNetCoreNlog

2020-01-12 Updated to .NET Core 3.1, NLog.Web.AspNetCore 4.9.0
2018-04-04 Updated to NLog.Web.AspNetCore 4.5.1, ASP.NET Core 2.0
2017.02.08 Updated to NLog.Web.AspNetCore 4.3.0 and VS2017 RC3
17.12.2016 Updated to ASP.NET Core 1.1

NLog posts in this series:

1. ASP.NET Core logging with NLog and Microsoft SQL Server
2. ASP.NET Core logging with NLog and Elasticsearch
3. Settings the NLog database connection string in the ASP.NET Core appsettings.json
4. .NET Core logging to MySQL using NLog
5. .NET Core logging with NLog and PostgreSQL

The XML nlog.config file is the same as in the previous post, with no database connection string configured.

<?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="Warn"
      internalLogFile="C:\git\damienbod\AspNetCoreNlog\Logs\internal-nlog.txt">

           
  <targets>

    <target name="database" xsi:type="Database" >

<!-- THIS is not required, read from the appsettings.json

<connectionString>${var:connectionString}</connectionString>
-->

<!--
  Remarks:
    The appsetting layouts require the NLog.Extended assembly.
    The aspnet-* layouts require the NLog.Web assembly.
    The Application value is determined by an AppName appSetting in Web.config.
    The "NLogDb" connection string determines the database that NLog write to.
    The create dbo.Log script in the comment below must be manually executed.

  Script for creating the dbo.Log table.

  SET ANSI_NULLS ON
  SET QUOTED_IDENTIFIER ON
  CREATE TABLE [dbo].[Log] (
      [Id] [int] IDENTITY(1,1) NOT NULL,
      [Application] [nvarchar](50) NOT NULL,
      [Logged] [datetime] NOT NULL,
      [Level] [nvarchar](50) NOT NULL,
      [Message] [nvarchar](max) NOT NULL,
      [Logger] [nvarchar](250) NULL,
      [Callsite] [nvarchar](max) NULL,
      [Exception] [nvarchar](max) NULL,
    CONSTRAINT [PK_dbo.Log] PRIMARY KEY CLUSTERED ([Id] ASC)
      WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
  ) ON [PRIMARY]
-->

          <commandText>
              insert into dbo.Log (
              Application, Logged, Level, Message,
              Logger, CallSite, Exception
              ) values (
              @Application, @Logged, @Level, @Message,
              @Logger, @Callsite, @Exception
              );
          </commandText>

          <parameter name="@application" layout="AspNetCoreNlog" />
          <parameter name="@logged" layout="${date}" />
          <parameter name="@level" layout="${level}" />
          <parameter name="@message" layout="${message}" />

          <parameter name="@logger" layout="${logger}" />
          <parameter name="@callSite" layout="${callsite:filename=true}" />
          <parameter name="@exception" layout="${exception:tostring}" />
      </target>
      
  </targets>

  <rules>
    <logger name="*" minlevel="Trace" writeTo="database" />
      
  </rules>
</nlog>

The NLog DatabaseTarget connectionstring is configured in the appsettings.json as described in the ASP.NET Core configuration docs.

{
    "Logging": {
        "IncludeScopes": false,
        "LogLevel": {
            "Default": "Debug",
            "System": "Information",
            "Microsoft": "Information"
        }
    },
    "ElasticsearchUrl": "http://localhost:9200",
    "ConnectionStrings": {
        "NLogDb": "Data Source=N275\\MSSQLSERVER2014;Initial Catalog=Nlogs;Integrated Security=True;"
    }
}

The configuration is then read in the Startup constructor.

public Startup(IConfiguration configuration)
{
	Configuration = configuration;
}

public IConfiguration Configuration { get; }

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
	services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
	// Add framework services.
	services.AddControllers();

	services.AddScoped<LogFilter>();
}

The Nlog DatabaseTagert is then configured to use the connection string from the app settings and sets all the DatabaseTarget instances for NLog to use this. All target properties can be configured in this way if required.

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
	GlobalDiagnosticsContext.Set("configDir", "C:\\git\\damienbod\\AspNetCoreNlog\\Logs");
	GlobalDiagnosticsContext.Set("connectionString", Configuration.GetConnectionString("DefaultConnection"));

	if (env.IsDevelopment())
	{
		app.UseDeveloperExceptionPage();
	}
	else
	{
		app.UseExceptionHandler("/Home/Error");
	}


	app.UseDefaultFiles();
	app.UseStaticFiles();

	app.UseRouting();

	app.UseEndpoints(endpoints =>
	{
		endpoints.MapControllerRoute(
			name: "default",
			pattern: "{controller=Home}/{action=Index}/{id?}");
	});
}

Links

https://github.com/NLog/NLog.Extensions.Logging

https://github.com/NLog

https://github.com/NLog/NLog/blob/38aef000f916bd5ffd8b80a5576afa2423192e84/examples/targets/Configuration%20API/Database/MSSQL/Example.cs

https://docs.asp.net/en/latest/fundamentals/logging.html

https://msdn.microsoft.com/en-us/magazine/mt694089.aspx

https://github.com/nlog/NLog/wiki/Database-target

https://docs.asp.net/en/latest/fundamentals/configuration.html