Microsoft.Extensions.Logging — ILogger, providers and configuration — Logging & Custom Logging in .NET — ILogger, Serilog, NLog, log4net and Your Own Logger

2. Microsoft.Extensions.Logging — ILogger, providers and configuration

Basic★ Must knowLesson 2 of 8

csharp

public class OrderService(ILogger<OrderService> logger)      // category = full class name
{
    public async Task PlaceAsync(Order order)
    {
        logger.LogInformation("Placing order {OrderId} with {LineCount} lines", order.Id, order.Lines.Count);
        try
        {
            await repository.SaveAsync(order);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, "Failed to save order {OrderId}", order.Id);   // exception first parameter
            throw;
        }
    }
}

Providers (where logs go): Console, Debug, EventSource, EventLog (Windows), Azure App Service, Application Insights, OpenTelemetry — plus third-party (Serilog, NLog) plugged in as providers.

csharp

var builder = WebApplication.CreateBuilder(args);    // adds Console, Debug, EventSource (+ EventLog on Windows) by default
builder.Logging.ClearProviders();
builder.Logging.AddConsole();
builder.Logging.AddJsonConsole();                    // structured JSON to stdout — ideal for containers

Filtering by category and level (appsettings.json):

json

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning",
      "Microsoft.EntityFrameworkCore.Database.Command": "Warning",
      "Divakar.Shop.Payments": "Debug"
    },
    "Console": { "LogLevel": { "Default": "Warning" } }
  }
}

The most specific category prefix wins; provider-specific sections override the general one; appsettings.Development.json usually turns on Debug.