Abstract [ Documentation]

ASP.NET Core supports logging with built-in and third-party logging providers: Console, Debug, Event Tracing, Windows Event Log, TraceSource, Azure App Service, Azure Application Insights.

Notes on Logging in .NET apply to these notes as well.
Documentation:
- HTTP Logging in .NET Core and ASP.NET Core | Microsoft Learn
- W3CLogger in .NET Core and ASP.NET Core | Microsoft Learn

Default Logging Providers

The default ASP.NET Core web app templates call WebApplication.CreateBuilder which adds the Console, Debug, EventSource and EventLog (Windows) logging providers.

Logs created with the default providers are displayed in:

Visual Studio’s Debug output window (when debugging)
Visual Studio’s ASP.NET Core Web Server window
The console window if running with dotnet run.

To change the logging providers:
Program.cs

var builder = WebApplication.CreateBuilder();
builder.Host.ConfigureLogging(logging =>
{
    logging.ClearProviders();
    logging.AddConsole();
});

Default log levels and all default providers:

{
  "Logging": {
    "LogLevel": { // No provider, LogLevel applies to all the enabled providers.
      "Default": "Error",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Warning"
    },
    "Debug": { // Debug provider.
      "LogLevel": {
        "Default": "Information" // Overrides preceding LogLevel:Default setting.
      }
    },
    "Console": {
      "IncludeScopes": true,
      "LogLevel": {
        "Microsoft.AspNetCore.Mvc.Razor.Internal": "Warning",
        "Microsoft.AspNetCore.Mvc.Razor.Razor": "Debug",
        "Microsoft.AspNetCore.Mvc.Razor": "Error",
        "Default": "Information"
      }
    },
    "EventSource": {
      "LogLevel": {
        "Microsoft": "Information"
      }
    },
    "EventLog": {
      "LogLevel": {
        "Microsoft": "Information"
      }
    },
    "AzureAppServicesFile": {
      "IncludeScopes": true,
      "LogLevel": {
        "Default": "Warning"
      }
    },
    "AzureAppServicesBlob": {
      "IncludeScopes": true,
      "LogLevel": {
        "Microsoft": "Information"
      }
    },
    "ApplicationInsights": {
      "LogLevel": {
        "Default": "Information"
      }
    }
  }
}

Creating Logs

To create logs, resolve an ILogger<TCategoryName> service from DI and call logging methods like LogInformation:

public class SomeClass {
    private readonly ILogger<SomeClass> _logger;
    
    public SomeMethod(ILogger<SomeClass> logger) {
        _logger = logger;
    }
    
    public async Task OnGetAsync() {
        _logger.LogInformation("SomeClass OnGetAsync.");
    }
}

Configure Logging

Logging configuration is provided by the Logging section of appsettings.ENVIRONMENT.json files:

appsettings.json

{
  "Logging": {
    "LogLevel": { // LogLevel applies to all the enabled providers.
      "Default": "Error", // Default logging, Error and higher.
      "Microsoft": "Warning" // All Microsoft* categories, Warning and higher.
    },
    "Debug": { // Debug provider.
      "LogLevel": {
        "Default": "Information", // Overrides preceding LogLevel:Default setting.
        "Microsoft.Hosting": "Trace" // Debug:Microsoft.Hosting category.
      }
    },
    "EventSource": { // EventSource provider.
      "LogLevel": {
        "Default": "Warning" // All categories of EventSource provider.
      }
    }
  }
}

The Logging property can contain LogLevel and logging provider properties. See Notes on Log Levels.

Creating Logs in Program.cs

The logger is available in app immediately after calling builder.Builder(): Program.cs

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

app.Logger.LogInformation("Adding Routes");
app.MapGet("/", () => "Hello World!");
app.Logger.LogInformation("Starting the app");
app.Run();

The loggers can also be modified before building:

var builder = WebApplication.CreateBuilder(args);

var logger = LoggerFactory.Create(config =>
{
    config.AddConsole();
}).CreateLogger("Program");

var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.MapGet("/Test", async context =>
{
    logger.LogInformation("Testing logging in Program.cs");
    await context.Response.WriteAsync("Testing");
});

app.Run();

ASP.NET Core and EF Core Logging Categories

Category	Notes
Microsoft.AspNetCore	General ASP.NET Core diagnostics.
Microsoft.AspNetCore.DataProtection	Which keys were considered, found, and used.
Microsoft.AspNetCore.HostFiltering	Hosts allowed.
Microsoft.AspNetCore.Hosting	How long HTTP requests took to complete and what time they started. Which hosting startup assemblies were loaded.
Microsoft.AspNetCore.Mvc	MVC and Razor diagnostics. Model binding, filter execution, view compilation, action selection.
Microsoft.AspNetCore.Routing	Route matching information.
Microsoft.AspNetCore.Server	Connection start, stop, and keep alive responses. HTTPS certificate information.
Microsoft.AspNetCore.StaticFiles	Files served.
Microsoft.EntityFrameworkCore	General Entity Framework Core diagnostics. Database activity and configuration, change detection, migrations.

Filtering

See these notes.

Log Categories

See these notes.

Log Event IDs

See these notes.

Log Message Templates

See these notes.

Exceptions

See these notes.

Scopes

See these notes.

ActivityTrackingOptions

Logging providers implicitly create a scope object with SpanId, TraceId, ParentId, Baggage and Tags. This is configured via ActivityTrackingOptions:

var loggerFactory = LoggerFactory.Create(logging =>
  {
      logging.Configure(options =>
      {
          options.ActivityTrackingOptions = ActivityTrackingOptions.SpanId
                                              | ActivityTrackingOptions.TraceId
                                              | ActivityTrackingOptions.ParentId
                                              | ActivityTrackingOptions.Baggage
                                              | ActivityTrackingOptions.Tags;
      }).AddSimpleConsole(options =>
      {
          options.IncludeScopes = true;
      });
  });

More information: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/logging/?view=aspnetcore-7.0#automatically-log-scope-with-spanid-traceid-parentid-baggage-and-tags