Mastering Logging in .NET Core Web API – With Hands-On Examples!

Logging in .NET Core Web API

Logging is the backbone of every well-built application. It helps track issues, debug errors, and monitor performance effortlessly. Whether you’re a beginner or an experienced developer, a well-structured logging system can save hours of debugging time.

1. Setting Up Logging in .NET Core Web API

Step 1: Create a .NET Core Web API Project

Open a terminal and run:

dotnet new webapi -n LoggingDemo

cd LoggingDemo

Step 2: Configure Logging in Program.cs

In .NET 6+, logging is configured inside Program.cs like this:

var builder = WebApplication.CreateBuilder(args);

 

//Configure built-in logging

builder.Logging.ClearProviders();  // Remove default logging providers

builder.Logging.AddConsole();      // Log to the console

builder.Logging.AddDebug();        // Log to Visual Studio Output Window

 

var app = builder.Build();

 

app.MapControllers();

app.Run();

Now, logs will appear in the console & Debug output!

 

2. Implementing Logging in Controllers

In ASP.NET Core, ILogger<T> is injected into controllers to enable logging.

Example: Add Logging in a Controller

using Microsoft.AspNetCore.Mvc;

using Microsoft.Extensions.Logging;

 

[ApiController]

[Route("[controller]")]

public class WeatherForecastController : ControllerBase

{

    private readonly ILogger<WeatherForecastController> _logger;

 

    public WeatherForecastController(ILogger<WeatherForecastController> logger)

    {

        _logger = logger;

    }

 

    [HttpGet]

    public IActionResult Get()

    {

        _logger.LogInformation("WeatherForecastController.Get() called");

 

        try

        {

            throw new Exception("Simulated exception!");

        }

        catch (Exception ex)

        {

            _logger.LogError(ex, "Error occurred while fetching data!");

        }

 

        return Ok(new { Message = "Check logs for details." });

    }

}

Logging Levels Explained

Log Level	Purpose
LogTrace()	Most detailed logs (for debugging only)
LogDebug()	Debugging logs (useful for local development)
LogInformation()	General application flow logs
LogWarning()	Logs potential issues that need attention
LogError()	Captures exceptions & errors
LogCritical()	Logs serious system failures

Run the API and check the logs in the console!

 

3. Configuring Logging in appsettings.json

By default, .NET Core allows you to configure logging in appsettings.json.

{

  "Logging": {

    "LogLevel": {

      "Default": "Information",

      "Microsoft": "Warning",

      "Microsoft.Hosting.Lifetime": "Information"

    }

  }

}

What Does This Mean?

Default: Logs Information and higher
Microsoft: Logs only Warning and higher (reduces clutter)

 

4. Logging to a File with Serilog

Although built-in logging is powerful, Serilog makes logging structured, persistent, and flexible.

Step 1: Install Serilog Packages

Run the following command:

dotnet add package Serilog.AspNetCore

dotnet add package Serilog.Sinks.File

Step 2: Configure Serilog in Program.cs

Modify Program.cs to use Serilog:

using Serilog;

 

var builder = WebApplication.CreateBuilder(args);

 

// Configure Serilog for structured logging

Log.Logger = new LoggerConfiguration()

    .WriteTo.Console()

    .WriteTo.File("logs/log.txt", rollingInterval: RollingInterval.Day)

    .CreateLogger();

 

builder.Host.UseSerilog();

 

var app = builder.Build();

 

app.MapControllers();

app.Run();

Now, all logs will be saved in logs/log.txt automatically!

 

5. Structured Logging with Serilog

Instead of plain text logs, structured logs make it easier to query and analyze data.

_logger.LogInformation("User {UserId} logged in at {Time}", userId, DateTime.UtcNow);

Output (JSON Format)

{

  "Message": "User 123 logged in at 2025-03-18T12:00:00Z",

  "UserId": 123,

  "Time": "2025-03-18T12:00:00Z"

}

Now, logs are searchable and filterable in tools like Seq or ELK Stack!

 

6. Centralized Logging with Seq (Real-Time Monitoring)

For real-time log monitoring, Seq is an excellent choice!

Step 1: Install Seq Sink

Run the command:

dotnet add package Serilog.Sinks.Seq

Step 2: Modify Program.cs

Log.Logger = new LoggerConfiguration()

    .WriteTo.Console()

    .WriteTo.Seq("http://localhost:5341")

    .CreateLogger();

Step 3: Start Seq with Docker

docker run --name seq -d -p 5341:80 datalust/seq

Now, all logs can be viewed in an interactive UI in Seq!

 

Conclusion

You’ve just built a fully functional logging system for your .NET Core Web API!

·       Built-in Logging with ILogger<T>

·       Configured Logging in appsettings.json

·       Saved Logs to Files with Serilog

·       Used Structured Logging for Better Analysis

·       Implemented Centralized Logging with Seq