ASP.NET Core Web API – Logging With NLog

Why is logging so important during the application development? Well, while your application is in the developing stage it is very easy to debug the code and to find out what went wrong. But, can you debug in the production environment?

Of course not. 

That is why logging messages are a great way to find out what went wrong and where did errors happen in your code in the production environment. .NET Core has its own implementation of logging messages, but in all my projects, I prefer to create my own custom logger service. 

If you want to see all the basic instructions and complete navigation for this series, please follow the following link: Introduction page for this tutorial.

For the previous part check out: Creating .NET Core WebApi project – Basic code preparations

This post is divided into several sections:

• Creating Required Projects
• Creating the Interface and Installing NLog
• Implementing Interface and Nlog.Config File
• Configuring Logger Service
• DI, IOC and Logger Service Testing
• Conclusion

Creating Required Projects

Let’s create two new projects. Name the first one Contracts. We are going to store interfaces inside this project. Name the second one LoggerService. We are going to use it for the logger logic.

To create a new project, right-click on the solution window, choose Add and then NewProject. We are going to choose the Class Library (.NET Core) and name it Contracts:

Do the same thing for the second project, just name it LoggerService.

With these two projects in place, we need to reference the LoggerService project to the main project. In the main project inside solution explorer, right-click on the Dependences and choose the AddReference. Under the Projects click the Solution and check this project:

Before we proceed to the implementation of the LoggerService, let’s do one more thing.

In the LoggerService project right click on the Dependencies and then click on the Add Reference. Inside check the Contracts checkbox to import its reference inside the LoggerService project. This would automatically add the Contracts reference to the main project because we already have a LoggerService referenced inside.

Creating the Interface and Installing NLog

Our logger service will contain four methods for logging:

• Info messages
• Debug messages
• Warning messages
• And error messages

Additionally, we are going to create the interface ILoggerManager inside the Contracts project containing those four method definitions.

So, let’s create the ILoggerManager interface and modify it:

namespace Contracts
{
    public interface ILoggerManager
    {
        void LogInfo(string message);
        void LogWarn(string message);
        void LogDebug(string message);
        void LogError(string message);
    }
}

Before we implement this interface inside the LoggerService project, we need to install the NLog library in our LoggerService project. NLog is a logging platform for the .NET which will help us create and log our massages.

We are going to show you two ways to add the NLog library to our project.

1. In the LoggerService project, right-click on the Dependences and choose Manage NuGet Packages. After the NuGet window appears, just follow the steps:

2. From the View menu choose the Other Windows and then click on the Package Manager Console. After the console appears just follow the leads:

Install-Package NLog.Extensions.Logging -Version 1.6.1

After a couple of seconds, NLog is up and running in your app.

Implementing Interface and Nlog.Config File

In the LoggerService project create the new class LoggerManager.

Let’s modify the LoggerManager class:

using Contracts;
using NLog;

namespace LoggerService
{
    public class LoggerManager : ILoggerManager
    {
        private static ILogger logger = LogManager.GetCurrentClassLogger();

        public void LogDebug(string message)
        {
            logger.Debug(message);
        }

        public void LogError(string message)
        {
            logger.Error(message);
        }

        public void LogInfo(string message)
        {
            logger.Info(message);
        }

        public void LogWarn(string message)
        {
            logger.Warn(message);
        }
    }
}

Now, we need to configure it and inject it into the Startup class in the ConfigureServices method.

NLog needs to have information about the folder to create log files in, what the name of these files will be and what a minimum level of logging is. Therefore, we need to create one text file in the main project with the name nlog.config and to populate it as in the example below. You need to change the path of the internal log and filename parameters to your paths.

<?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="Trace"
      internalLogFile="d:Projects\Blog-AccountOwner\Project\internal_logs\internallog.txt">

  <targets>
    <target name="logfile" xsi:type="File"
            fileName="d:/Projects/Blog-AccountOwner/Project/logs/${shortdate}_logfile.txt"
            layout="${longdate} ${level:uppercase=true} ${message}"/>
  </targets>

  <rules>
    <logger name="*" minlevel="Debug" writeTo="logfile" />
  </rules>
</nlog>

Configuring Logger Service for Logging Messages

Setting up the configuration for a logger service is quite easy.

We have to modify the constructor of the Startup class:

public Startup(IConfiguration configuration)
{
    LogManager.LoadConfiguration(String.Concat(Directory.GetCurrentDirectory(), "/nlog.config"));
    Configuration = configuration;
}

Secondly, we need to add the logger service inside the .NET Core’s IOC container. There are three ways to do that:

• By calling services.AddSingleton will create the service the first time you request it and then every subsequent request is calling the same instance of the service. This means that all components are sharing the same service every time they need it. You are using the same instance always
• By calling services.AddScoped will create the service once per request. That means whenever we send the HTTP request towards the application, a new instance of the service is created
• By calling services.AddTransient will create the service each time the application request it. This means that if during one request towards our application, multiple components need the service, this service will be created again for every single component which needs it

So, in the ServiceExtensions class we are going to add a new method:

public static void ConfigureLoggerService(this IServiceCollection services)
{
    services.AddSingleton<ILoggerManager, LoggerManager>();
}

Lastly, in the ConfigureServices method invoke this extension method, right above services.AddControllers():

services.ConfigureLoggerService();

Every time we want to use a logger service, all we need to do is to inject it into the constructor of the class that is going to use that service. .NET Core will serve that service from the IOC container and all of its features will be available to use. This type of injecting objects is called Dependency Injection.

DI, IOC and Logger Service Testing

What is exactly Dependency Injection(DI) and what is IOC (Inversion of Control)?

Dependency injection is a technique for achieving loose coupling between objects and their dependencies. It means that rather than instantiating an object every time it is needed, we can instantiate it once and then serve it in a class, most often, through a constructor method. This specific approach we utilize is also known as the Constructor Injection.

In a system that is designed to use DI, you may find many classes requesting their dependencies via their constructor. In that case, it is helpful to have a class that will provide all instances to classes through the constructor. These classes are referred to as containers or more specific Inversion of Control containers. An IOC container is essentially a factory that is responsible for providing instances of types that are requested from it.

For the logger service testing purpose, we could use WeatherForecastController. We are going to find it in the main project in the Controllers folder.

So, let’s modify it:

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    private readonly ILoggerManager _logger;

    public WeatherForecastController(ILoggerManager logger)
    {
        _logger = logger;
    }

    [HttpGet]
    public IEnumerable<string> Get()
    {
        _logger.LogInfo("Here is info message from the controller.");
        _logger.LogDebug("Here is debug message from the controller.");
        _logger.LogWarn("Here is warn message from the controller.");
        _logger.LogError("Here is error message from the controller.");

        return new string[] { "value1", "value2" };
    }
}

After all, start the application, and browse to http://localhost:5000/weatherforecast. As a result, you will see an array of two strings. Now go to the folder that you have specified in the nlog.config file, and check out the result. You should see four lines logged inside that file.

Conclusion

Now you have the working logger service, you will use through an entire process of our application development.

By reading this post you’ve learned:

• Why logging is important
• How to create external service
• How to use NLog library and how to configure it
• What are DI and IOC
• And how to use them for service injection

Thank you for reading and see you soon in the next blog post where we will talk about repository pattern with entity framework core.