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

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.

1️⃣ Setting Up Logging in .NET Core

📦 Step 1: Create a Project

dotnet new webapi -n LoggingDemo
cd LoggingDemo

🛠️ Step 2: Configure Logging in Program.cs

var builder = WebApplication.CreateBuilder(args);

builder.Logging.ClearProviders();   // Remove default logging
builder.Logging.AddConsole();       // Console logs
builder.Logging.AddDebug();         // Debug window logs

var app = builder.Build();

app.MapControllers();
app.Run();

✅ Logs will now appear in the terminal and Visual Studio output!

2️⃣ Implementing Logging in Controllers

✏️ Example: Injecting ILogger

[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

Level	Description
LogTrace()	Most detailed logs
LogDebug()	Useful for local debugging
LogInformation()	Normal app behavior
LogWarning()	Potential issues
LogError()	Exceptions and errors
LogCritical()	System crash or critical failure

3️⃣ Configuring Logging in appsettings.json

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  }
}

📘 Explanation:
Default logs everything from Information upwards.
Microsoft filters out lower-level logs to avoid clutter.

4️⃣ Logging to File with Serilog

📦 Install Serilog Packages

dotnet add package Serilog.AspNetCore
dotnet add package Serilog.Sinks.File

🔧 Configure in Program.cs

using Serilog;

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console()
    .WriteTo.File("logs/log.txt", rollingInterval: RollingInterval.Day)
    .CreateLogger();

builder.Host.UseSerilog();

5️⃣ Structured Logging with Serilog

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

🎯 Output (structured JSON):
{ "Message": "User 123 logged in at 2025-03-18T12:00:00Z" }
Structured logs help with filtering and searching in tools like Seq or ELK.

6️⃣ Centralized Logging with Seq

📦 Install Seq Sink

dotnet add package Serilog.Sinks.Seq

🔧 Configure Seq Sink in Program.cs

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console()
    .WriteTo.Seq("http://localhost:5341")
    .CreateLogger();

🐳 Start Seq via Docker

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

📊 You can now monitor logs in a real-time dashboard using Seq!

🧠 Conclusion

• ✅ Used ILogger<T> for built-in logging
• ✅ Configured log levels via appsettings.json
• ✅ Saved logs to files using Serilog
• ✅ Used structured logging for searchability
• ✅ Integrated Seq for centralized, real-time monitoring