Introduction To NLog With ASP.NET Core

PM> Install-Package NLog  
PM> Install-Package NLog.Web.AspNetCore  

<ItemGroup>  
  <PackageReference Include="Microsoft.AspNetCore.App" />  
  <PackageReference Include="Microsoft.AspNetCore.Razor.Design" Version="2.1.2" PrivateAssets="All" />  
  <PackageReference Include="NLog.Web.AspNetCore" Version="4.5.4" />  
  <PackageReference Include="NLog" Version="4.5.4" />  
</ItemGroup>

<?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="internalLog.txt">  
    <extensions>  
        <add assembly="NLog.Web.AspNetCore" />  
    </extensions>  
    <!-- the targets to write to -->  
    <targets>  
        <!-- write to file -->  
        <target xsi:type="File" name="alldata" fileName="demo-${shortdate}.log" layout="${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}" />  
        <!-- another file log. Uses some ASP.NET core renderers -->  
        <target xsi:type="File" name="otherFile-web" fileName="demo-Other-${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>  
        <logger name="*" minlevel="Trace" writeTo="alldata" />  
        <!--Skip non-critical Microsoft logs and so log only own logs-->  
        <logger name="Microsoft.*" maxLevel="Info" final="true" />  
        <loggername="*"minlevel="Trace"writeTo="otherFile-web" />  
    </rules>  
</nlog>

This file must be available at a place where project's DLL is placed. So, we need to enable the "copy to bin" folder.

Step 3. Configure NLog in application

Using "UseNLog" method, we can add NLog in a request pipeline as a dependency. To catch an error in program class, we can set up a logger class and after initialization of program class, we can safely shut down the log manager.

public class Program  
{  
    public static void Main(string[] args)  
    {  
        var logger = NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger();  
        try  
        {  
            logger.Debug("init main function");  
            CreateWebHostBuilder(args).Build().Run();  
        }  
        catch (Exception ex)  
        {  
            logger.Error(ex, "Error in init");  
            throw;  
        }  
        finally  
        {  
            NLog.LogManager.Shutdown();  
        }  
              
    }  
  
    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>  
        WebHost.CreateDefaultBuilder(args)  
            .UseStartup<Startup>()  
            .ConfigureLogging(logging =>  
            {  
                logging.ClearProviders();  
                logging.SetMinimumLevel(LogLevel.Information);  
            })  
            .UseNLog();  
}

In the above code, we are also configuring the logging (i.e. SetMinimumLevel). This can be overridden by the appsettings.json file. So, we need to adjust it correctly as we need.

Demo

As we know, Logger is available as DI (dependency injection) to every controller by default, we get an ILogger object in the constructor of the controller class.

public class HomeController : Controller  
{  
    private readonly ILogger<HomeController> _logger;  
  
    public HomeController(ILogger<HomeController> logger)  
    {  
        _logger = logger;  
    }  
    public IActionResult Index()  
    {  
        _logger.LogInformation("HomeController.Index method called!!!");  
        return View();  
    }  
}

Output

The file format can be defined using the layout attribute in a target element.

Summary

NLog is an open source framework for logging. It is easy to configure with ASP.NET Core application. In this article, I have explained about NLog - file logging configuration with ASP.NET Core 2.

You can view or download the source code from the GitHub link here.