Overview

• Built-in providers let you send logs to one or more destinations, and you can plug in a third-party logging framework.
• If your data store is slow, write the log messages to a fast store first, then move them to a slow store later.

public class TodoController : Controller
{
    private readonly ILogger _logger;

    public TodoController(ILogger<TodoController> logger)
    {
				_logger = logger;
    }
}

• To use a logger provider, add their service in the ConfigureService method.

public static void Main(string[] args)
{
    var webHost = new WebHostBuilder()
        .UseKestrel()
        .UseContentRoot(Directory.GetCurrentDirectory())
        .ConfigureAppConfiguration((hostingContext, config) =>
        {
            var env = hostingContext.HostingEnvironment;
            config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                  .AddJsonFile($"appsettings.{env.EnvironmentName}.json", 
										optional: true, reloadOnChange: true);
        })
        .ConfigureLogging((hostingContext, logging) =>
        {
            logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
            logging.AddConsole();
            logging.AddDebug();
        })
        .UseStartup<Startup>()
        .Build();

    webHost.Run();
}

• The default project template enables logging with the CreateDefaultBuilder method:

public static void Main(string[] args)
{
    BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .Build();

Log Factory and Log Category

• A category is included with each log that you create. You specify the category when you create an ILogger object.
• The category may be any string, but a convention is to use the fully qualified name of the class from which the logs are written.
• You can specify the category as a string or use an extension method that derives the category from the type.
• To specify the category as a string, call CreateLogger on an ILoggerFactory instance.
• This is equivalent to calling CreateLogger with the fully qualified type name of T.

public class TodoController : Controller
{
    private readonly ILogger _logger;

    public TodoController(ILoggerFactory logger)
    {
        _logger = logger.CreateLogger("TodoApi.Controllers.TodoController");
    }
}

Log Levels

• The log level indicates the degree of severity or importance.
• You might write an Information log when a method ends normally, a Warning log when a method returns a 404 return code, and an Error log when you catch an unexpected exception.

Trace = 0

• For information that's valuable only to a developer debugging an issue.