Logging in ASP.NET Core

By Steve Smith and Tom Dykstra

ASP.NET Core supports a logging API that works with a variety of built-in and third-party logging providers. This article shows how to use the logging API with built-in providers.

View or download sample code (how to download)

Add providers

A logging provider displays or stores logs. For example, the Console provider displays logs on the console, and the Azure Application Insights provider stores them in Azure Application Insights. Logs can be sent to multiple destinations by adding multiple providers.

To add a provider, call the provider's Add{provider name} extension method in Program.cs:

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);
            config.AddEnvironmentVariables();
        })
        .ConfigureLogging((hostingContext, logging) =>
        {
            logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
            logging.AddConsole();
            logging.AddDebug();
            logging.AddEventSourceLogger();
        })
        .UseStartup<Startup>()
        .Build();

    webHost.Run();
}

The default project template calls the CreateDefaultBuilder extension method, which adds the following logging providers:

  • Console
  • Debug
  • EventSource (starting in ASP.NET Core 2.2)
public static void Main(string[] args)
{
    BuildWebHost(args).Run();
}

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

If you use CreateDefaultBuilder, you can replace the default providers with your own choices. Call ClearProviders, and add the providers you want.

public static void Main(string[] args)
{
    var host = BuildWebHost(args);

    var todoRepository = host.Services.GetRequiredService<ITodoRepository>();
    todoRepository.Add(new Core.Model.TodoItem() { Name = "Feed the dog" });
    todoRepository.Add(new Core.Model.TodoItem() { Name = "Walk the dog" });

    var logger = host.Services.GetRequiredService<ILogger<Program>>();
    logger.LogInformation("Seeded the database.");

    host.Run();
}

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .ConfigureLogging(logging =>
        {
            logging.ClearProviders();
            logging.AddConsole();
        })
        .Build();

To use a provider, install its NuGet package and call the provider's extension method on an instance of ILoggerFactory:

public void Configure(IApplicationBuilder app,
    IHostingEnvironment env,
    ILoggerFactory loggerFactory)
{
    loggerFactory
        .AddConsole()
        .AddDebug();

ASP.NET Core dependency injection (DI) provides the ILoggerFactory instance. The AddConsole and AddDebug extension methods are defined in the Microsoft.Extensions.Logging.Console and Microsoft.Extensions.Logging.Debug packages. Each extension method calls the ILoggerFactory.AddProvider method, passing in an instance of the provider.

Note

The sample app adds logging providers in the Startup.Configure method. To obtain log output from code that executes earlier, add logging providers in the Startup class constructor.

Learn more about built-in logging providers and third-party logging providers later in the article.

Create logs

Get an ILogger<TCategoryName> object from DI.

The following controller example creates Information and Warning logs. The category is TodoApiSample.Controllers.TodoController (the fully qualified class name of TodoController in the sample app):

public class TodoController : Controller
{
    private readonly ITodoRepository _todoRepository;
    private readonly ILogger _logger;

    public TodoController(ITodoRepository todoRepository,
        ILogger<TodoController> logger)
    {
        _todoRepository = todoRepository;
        _logger = logger;
    }
public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}

The following Razor Pages example creates logs with Information as the level and TodoApiSample.Pages.AboutModel as the category:

public class AboutModel : PageModel
{
    private readonly ILogger _logger;

    public AboutModel(ILogger<AboutModel> logger)
    {
        _logger = logger;
    }
public void OnGet()
{
    Message = $"About page visited at {DateTime.UtcNow.ToLongTimeString()}";
    _logger.LogInformation("Message displayed: {Message}", Message);
}
public class TodoController : Controller
{
    private readonly ITodoRepository _todoRepository;
    private readonly ILogger _logger;

    public TodoController(ITodoRepository todoRepository,
        ILogger<TodoController> logger)
    {
        _todoRepository = todoRepository;
        _logger = logger;
    }
public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}

The preceding example creates logs with Information and Warning as the level and TodoController class as the category.

The Log level indicates the severity of the logged event. The log category is a string that is associated with each log. The ILogger<T> instance creates logs that have the fully qualified name of type T as the category. Levels and categories are explained in more detail later in this article.

Create logs in Startup

To write logs in the Startup class, include an ILogger parameter in the constructor signature:

public class Startup
{
    private readonly ILogger _logger;

    public Startup(IConfiguration configuration, ILogger<Startup> logger)
    {
        Configuration = configuration;
        _logger = logger;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();

        // Add our repository type
        services.AddSingleton<ITodoRepository, TodoRepository>();
        _logger.LogInformation("Added TodoRepository to services");
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            _logger.LogInformation("In Development environment");
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Error");
            app.UseHsts();
        }

        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseCookiePolicy();

        app.UseMvc();
    }
}

Create logs in Program

To write logs in the Program class, get an ILogger instance from DI:

public static void Main(string[] args)
{
    var host = BuildWebHost(args);

    var todoRepository = host.Services.GetRequiredService<ITodoRepository>();
    todoRepository.Add(new Core.Model.TodoItem() { Name = "Feed the dog" });
    todoRepository.Add(new Core.Model.TodoItem() { Name = "Walk the dog" });

    var logger = host.Services.GetRequiredService<ILogger<Program>>();
    logger.LogInformation("Seeded the database.");

    host.Run();
}

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .ConfigureLogging(logging =>
        {
            logging.ClearProviders();
            logging.AddConsole();
        })
        .Build();

No asynchronous logger methods

Logging should be so fast that it isn't worth the performance cost of asynchronous code. If your logging data store is slow, don't write to it directly. Consider writing the log messages to a fast store initially, then move them to the slow store later. For example, log to a message queue that's read and persisted to slow storage by another process.

Configuration

Logging provider configuration is provided by one or more configuration providers:

  • File formats (INI, JSON, and XML).
  • Command-line arguments.
  • Environment variables.
  • In-memory .NET objects.
  • The unencrypted Secret Manager storage.
  • An encrypted user store, such as Azure Key Vault.
  • Custom providers (installed or created).

For example, logging configuration is commonly provided by the Logging section of app settings files. The following example shows the contents of a typical appsettings.Development.json file:

{
  "Logging": {
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    },
    "Console":
    {
      "IncludeScopes": true
    }
  }
}

The Logging property can have LogLevel and log provider properties (Console is shown).

The LogLevel property under Logging specifies the minimum level to log for selected categories. In the example, System and Microsoft categories log at Information level, and all others log at Debug level.

Other properties under Logging specify logging providers. The example is for the Console provider. If a provider supports log scopes, IncludeScopes indicates whether they're enabled. A provider property (such as Console in the example) may also specify a LogLevel property. LogLevel under a provider specifies levels to log for that provider.

If levels are specified in Logging.{providername}.LogLevel, they override anything set in Logging.LogLevel.

{
  "Logging": {
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

LogLevel keys represent log names. The Default key applies to logs not explicitly listed. The value represents the log level applied to the given log.

For information on implementing configuration providers, see Configuration in ASP.NET Core.

Sample logging output

With the sample code shown in the preceding section, logs appear in the console when the app is run from the command line. Here's an example of console output:

info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
      Request starting HTTP/1.1 GET http://localhost:5000/api/todo/0
info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[1]
      Executing action method TodoApi.Controllers.TodoController.GetById (TodoApi) with arguments (0) - ModelState is Valid
info: TodoApi.Controllers.TodoController[1002]
      Getting item 0
warn: TodoApi.Controllers.TodoController[4000]
      GetById(0) NOT FOUND
info: Microsoft.AspNetCore.Mvc.StatusCodeResult[1]
      Executing HttpStatusCodeResult, setting HTTP status code 404
info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[2]
      Executed action TodoApi.Controllers.TodoController.GetById (TodoApi) in 42.9286ms
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
      Request finished in 148.889ms 404

The preceding logs were generated by making an HTTP Get request to the sample app at http://localhost:5000/api/todo/0.

Here's an example of the same logs as they appear in the Debug window when you run the sample app in Visual Studio:

Microsoft.AspNetCore.Hosting.Internal.WebHost:Information: Request starting HTTP/1.1 GET http://localhost:53104/api/todo/0  
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker:Information: Executing action method TodoApi.Controllers.TodoController.GetById (TodoApi) with arguments (0) - ModelState is Valid
TodoApi.Controllers.TodoController:Information: Getting item 0
TodoApi.Controllers.TodoController:Warning: GetById(0) NOT FOUND
Microsoft.AspNetCore.Mvc.StatusCodeResult:Information: Executing HttpStatusCodeResult, setting HTTP status code 404
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker:Information: Executed action TodoApi.Controllers.TodoController.GetById (TodoApi) in 152.5657ms
Microsoft.AspNetCore.Hosting.Internal.WebHost:Information: Request finished in 316.3195ms 404

The logs that are created by the ILogger calls shown in the preceding section begin with "TodoApi.Controllers.TodoController". The logs that begin with "Microsoft" categories are from ASP.NET Core framework code. ASP.NET Core and application code are using the same logging API and providers.

The remainder of this article explains some details and options for logging.

NuGet packages

The ILogger and ILoggerFactory interfaces are in Microsoft.Extensions.Logging.Abstractions, and default implementations for them are in Microsoft.Extensions.Logging.

Log category

When an ILogger object is created, a category is specified for it. That category is included with each log message created by that instance of Ilogger. The category may be any string, but the convention is to use the class name, such as "TodoApi.Controllers.TodoController".

Use ILogger<T> to get an ILogger instance that uses the fully qualified type name of T as the category:

public class TodoController : Controller
{
    private readonly ITodoRepository _todoRepository;
    private readonly ILogger _logger;

    public TodoController(ITodoRepository todoRepository,
        ILogger<TodoController> logger)
    {
        _todoRepository = todoRepository;
        _logger = logger;
    }
public class TodoController : Controller
{
    private readonly ITodoRepository _todoRepository;
    private readonly ILogger _logger;

    public TodoController(ITodoRepository todoRepository,
        ILogger<TodoController> logger)
    {
        _todoRepository = todoRepository;
        _logger = logger;
    }

To explicitly specify the category, call ILoggerFactory.CreateLogger:

public class TodoController : Controller
{
    private readonly ITodoRepository _todoRepository;
    private readonly ILogger _logger;

    public TodoController(ITodoRepository todoRepository,
        ILoggerFactory logger)
    {
        _todoRepository = todoRepository;
        _logger = logger.CreateLogger("TodoApiSample.Controllers.TodoController");
    }
public class TodoController : Controller
{
    private readonly ITodoRepository _todoRepository;
    private readonly ILogger _logger;

    public TodoController(ITodoRepository todoRepository,
        ILoggerFactory logger)
    {
        _todoRepository = todoRepository;
        _logger = logger.CreateLogger("TodoApiSample.Controllers.TodoController");
    }

ILogger<T> is equivalent to calling CreateLogger with the fully qualified type name of T.

Log level

Every log specifies a LogLevel value. The log level indicates the severity or importance. For example, you might write an Information log when a method ends normally and a Warning log when a method returns a 404 Not Found status code.

The following code creates Information and Warning logs:

public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}
public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}

In the preceding code, the first parameter is the Log event ID. The second parameter is a message template with placeholders for argument values provided by the remaining method parameters. The method parameters are explained in the message template section later in this article.

Log methods that include the level in the method name (for example, LogInformation and LogWarning) are extension methods for ILogger. These methods call a Log method that takes a LogLevel parameter. You can call the Log method directly rather than one of these extension methods, but the syntax is relatively complicated. For more information, see ILogger and the logger extensions source code.

ASP.NET Core defines the following log levels, ordered here from lowest to highest severity.

  • Trace = 0

    For information that's typically valuable only for debugging. These messages may contain sensitive application data and so shouldn't be enabled in a production environment. Disabled by default.

  • Debug = 1

    For information that may be useful in development and debugging. Example: Entering method Configure with flag set to true. Enable Debug level logs in production only when troubleshooting, due to the high volume of logs.

  • Information = 2

    For tracking the general flow of the app. These logs typically have some long-term value. Example: Request received for path /api/todo

  • Warning = 3

    For abnormal or unexpected events in the app flow. These may include errors or other conditions that don't cause the app to stop but might need to be investigated. Handled exceptions are a common place to use the Warning log level. Example: FileNotFoundException for file quotes.txt.

  • Error = 4

    For errors and exceptions that cannot be handled. These messages indicate a failure in the current activity or operation (such as the current HTTP request), not an app-wide failure. Example log message: Cannot insert record due to duplicate key violation.

  • Critical = 5

    For failures that require immediate attention. Examples: data loss scenarios, out of disk space.

Use the log level to control how much log output is written to a particular storage medium or display window. For example:

  • In production, send Trace through Information level to a volume data store. Send Warning through Critical to a value data store.
  • During development, send Warning through Critical to the console, and add Trace through Information when troubleshooting.

The Log filtering section later in this article explains how to control which log levels a provider handles.

ASP.NET Core writes logs for framework events. The log examples earlier in this article excluded logs below Information level, so no Debug or Trace level logs were created. Here's an example of console logs produced by running the sample app configured to show Debug logs:

info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
      Request starting HTTP/1.1 GET http://localhost:62555/api/todo/0
dbug: Microsoft.AspNetCore.Routing.Tree.TreeRouter[1]
      Request successfully matched the route with name 'GetTodo' and template 'api/Todo/{id}'.
dbug: Microsoft.AspNetCore.Mvc.Internal.ActionSelector[2]
      Action 'TodoApi.Controllers.TodoController.Update (TodoApi)' with id '089d59b6-92ec-472d-b552-cc613dfd625d' did not match the constraint 'Microsoft.AspNetCore.Mvc.Internal.HttpMethodActionConstraint'
dbug: Microsoft.AspNetCore.Mvc.Internal.ActionSelector[2]
      Action 'TodoApi.Controllers.TodoController.Delete (TodoApi)' with id 'f3476abe-4bd9-4ad3-9261-3ead09607366' did not match the constraint 'Microsoft.AspNetCore.Mvc.Internal.HttpMethodActionConstraint'
dbug: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[1]
      Executing action TodoApi.Controllers.TodoController.GetById (TodoApi)
info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[1]
      Executing action method TodoApi.Controllers.TodoController.GetById (TodoApi) with arguments (0) - ModelState is Valid
info: TodoApi.Controllers.TodoController[1002]
      Getting item 0
warn: TodoApi.Controllers.TodoController[4000]
      GetById(0) NOT FOUND
dbug: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[2]
      Executed action method TodoApi.Controllers.TodoController.GetById (TodoApi), returned result Microsoft.AspNetCore.Mvc.NotFoundResult.
info: Microsoft.AspNetCore.Mvc.StatusCodeResult[1]
      Executing HttpStatusCodeResult, setting HTTP status code 404
info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[2]
      Executed action TodoApi.Controllers.TodoController.GetById (TodoApi) in 0.8788ms
dbug: Microsoft.AspNetCore.Server.Kestrel[9]
      Connection id "0HL6L7NEFF2QD" completed keep alive response.
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
      Request finished in 2.7286ms 404

Log event ID

Each log can specify an event ID. The sample app does this by using a locally defined LoggingEvents class:

public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}
public class LoggingEvents
{
    public const int GenerateItems = 1000;
    public const int ListItems = 1001;
    public const int GetItem = 1002;
    public const int InsertItem = 1003;
    public const int UpdateItem = 1004;
    public const int DeleteItem = 1005;

    public const int GetItemNotFound = 4000;
    public const int UpdateItemNotFound = 4001;
}
public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}
public class LoggingEvents
{
    public const int GenerateItems = 1000;
    public const int ListItems = 1001;
    public const int GetItem = 1002;
    public const int InsertItem = 1003;
    public const int UpdateItem = 1004;
    public const int DeleteItem = 1005;

    public const int GetItemNotFound = 4000;
    public const int UpdateItemNotFound = 4001;
}

An event ID associates a set of events. For example, all logs related to displaying a list of items on a page might be 1001.

The logging provider may store the event ID in an ID field, in the logging message, or not at all. The Debug provider doesn't show event IDs. The console provider shows event IDs in brackets after the category:

info: TodoApi.Controllers.TodoController[1002]
      Getting item invalidid
warn: TodoApi.Controllers.TodoController[4000]
      GetById(invalidid) NOT FOUND

Log message template

Each log specifies a message template. The message template can contain placeholders for which arguments are provided. Use names for the placeholders, not numbers.

public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}
public IActionResult GetById(string id)
{
    _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
    var item = _todoRepository.Find(id);
    if (item == null)
    {
        _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
        return NotFound();
    }
    return new ObjectResult(item);
}

The order of placeholders, not their names, determines which parameters are used to provide their values. In the following code, notice that the parameter names are out of sequence in the message template:

string p1 = "parm1";
string p2 = "parm2";
_logger.LogInformation("Parameter values: {p2}, {p1}", p1, p2);

This code creates a log message with the parameter values in sequence:

Parameter values: parm1, parm2

The logging framework works this way so that logging providers can implement semantic logging, also known as structured logging. The arguments themselves are passed to the logging system, not just the formatted message template. This information enables logging providers to store the parameter values as fields. For example, suppose logger method calls look like this:

_logger.LogInformation("Getting item {ID} at {RequestTime}", id, DateTime.Now);

If you're sending the logs to Azure Table Storage, each Azure Table entity can have ID and RequestTime properties, which simplifies queries on log data. A query can find all logs within a particular RequestTime range without parsing the time out of the text message.

Logging exceptions

The logger methods have overloads that let you pass in an exception, as in the following example:

catch (Exception ex)
{
    _logger.LogWarning(LoggingEvents.GetItemNotFound, ex, "GetById({ID}) NOT FOUND", id);
    return NotFound();
}
return new ObjectResult(item);
catch (Exception ex)
{
    _logger.LogWarning(LoggingEvents.GetItemNotFound, ex, "GetById({ID}) NOT FOUND", id);
    return NotFound();
}
return new ObjectResult(item);

Different providers handle the exception information in different ways. Here's an example of Debug provider output from the code shown above.

TodoApi.Controllers.TodoController:Warning: GetById(036dd898-fb01-47e8-9a65-f92eb73cf924) NOT FOUND

System.Exception: Item not found exception.
 at TodoApi.Controllers.TodoController.GetById(String id) in C:\logging\sample\src\TodoApi\Controllers\TodoController.cs:line 226

Log filtering

You can specify a minimum log level for a specific provider and category or for all providers or all categories. Any logs below the minimum level aren't passed to that provider, so they don't get displayed or stored.

To suppress all logs, specify LogLevel.None as the minimum log level. The integer value of LogLevel.None is 6, which is higher than LogLevel.Critical (5).

Create filter rules in configuration

The project template code calls CreateDefaultBuilder to set up logging for the Console and Debug providers. The CreateDefaultBuilder method also sets up logging to look for configuration in a Logging section, using code like the following:

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);
            config.AddEnvironmentVariables();
        })
        .ConfigureLogging((hostingContext, logging) =>
        {
            logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
            logging.AddConsole();
            logging.AddDebug();
            logging.AddEventSourceLogger();
        })
        .UseStartup<Startup>()
        .Build();

    webHost.Run();
}

The configuration data specifies minimum log levels by provider and category, as in the following example:

{
  "Logging": {
    "Debug": {
      "LogLevel": {
        "Default": "Information"
      }
    },
    "Console": {
      "IncludeScopes": false,
      "LogLevel": {
        "Microsoft.AspNetCore.Mvc.Razor.Internal": "Warning",
        "Microsoft.AspNetCore.Mvc.Razor.Razor": "Debug",
        "Microsoft.AspNetCore.Mvc.Razor": "Error",
        "Default": "Information"
      }
    },
    "LogLevel": {
      "Default": "Debug"
    }
  }
}

This JSON creates six filter rules: one for the Debug provider, four for the Console provider, and one for all providers. A single rule is chosen for each provider when an ILogger object is created.

Filter rules in code

The following example shows how to register filter rules in code:

WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>()
    .ConfigureLogging(logging =>
        logging.AddFilter("System", LogLevel.Debug)
               .AddFilter<DebugLoggerProvider>("Microsoft", LogLevel.Trace))
    .Build();

The second AddFilter specifies the Debug provider by using its type name. The first AddFilter applies to all providers because it doesn't specify a provider type.

How filtering rules are applied

The configuration data and the AddFilter code shown in the preceding examples create the rules shown in the following table. The first six come from the configuration example and the last two come from the code example.

Number Provider Categories that begin with ... Minimum log level
1 Debug All categories Information
2 Console Microsoft.AspNetCore.Mvc.Razor.Internal Warning
3 Console Microsoft.AspNetCore.Mvc.Razor.Razor Debug
4 Console Microsoft.AspNetCore.Mvc.Razor Error
5 Console All categories Information
6 All providers All categories Debug
7 All providers System Debug
8 Debug Microsoft Trace

When an ILogger object is created, the ILoggerFactory object selects a single rule per provider to apply to that logger. All messages written by an ILogger instance are filtered based on the selected rules. The most specific rule possible for each provider and category pair is selected from the available rules.

The following algorithm is used for each provider when an ILogger is created for a given category:

  • Select all rules that match the provider or its alias. If no match is found, select all rules with an empty provider.
  • From the result of the preceding step, select rules with longest matching category prefix. If no match is found, select all rules that don't specify a category.
  • If multiple rules are selected, take the last one.
  • If no rules are selected, use MinimumLevel.

With the preceding list of rules, suppose you create an ILogger object for category "Microsoft.AspNetCore.Mvc.Razor.RazorViewEngine":

  • For the Debug provider, rules 1, 6, and 8 apply. Rule 8 is most specific, so that's the one selected.
  • For the Console provider, rules 3, 4, 5, and 6 apply. Rule 3 is most specific.

The resulting ILogger instance sends logs of Trace level and above to the Debug provider. Logs of Debug level and above are sent to the Console provider.

Provider aliases

Each provider defines an alias that can be used in configuration in place of the fully qualified type name. For the built-in providers, use the following aliases:

  • Console
  • Debug
  • EventLog
  • AzureAppServices
  • TraceSource
  • EventSource

Default minimum level

There's a minimum level setting that takes effect only if no rules from configuration or code apply for a given provider and category. The following example shows how to set the minimum level:

WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>()
    .ConfigureLogging(logging => logging.SetMinimumLevel(LogLevel.Warning))
    .Build();

If you don't explicitly set the minimum level, the default value is Information, which means that Trace and Debug logs are ignored.

Filter functions

A filter function is invoked for all providers and categories that don't have rules assigned to them by configuration or code. Code in the function has access to the provider type, category, and log level. For example:

WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>()
    .ConfigureLogging(logBuilder =>
    {
        logBuilder.AddFilter((provider, category, logLevel) =>
        {
            if (provider == "Microsoft.Extensions.Logging.Console.ConsoleLoggerProvider" && 
                category == "TodoApiSample.Controllers.TodoController")
            {
                return false;
            }
            return true;
        });
    })
    .Build();

Some logging providers let you specify when logs should be written to a storage medium or ignored based on log level and category.

The AddConsole and AddDebug extension methods provide overloads that accept filtering criteria. The following sample code causes the console provider to ignore logs below Warning level, while the Debug provider ignores logs that the framework creates.

public void Configure(IApplicationBuilder app,
    IHostingEnvironment env,
    ILoggerFactory loggerFactory)
{
    loggerFactory
        .AddConsole(LogLevel.Warning)
        .AddDebug((category, logLevel) => (category.Contains("TodoApi") && logLevel >= LogLevel.Trace));

The AddEventLog method has an overload that takes an EventLogSettings instance, which may contain a filtering function in its Filter property. The TraceSource provider doesn't provide any of those overloads, since its logging level and other parameters are based on the SourceSwitch and TraceListener it uses.

To set filtering rules for all providers that are registered with an ILoggerFactory instance, use the WithFilter extension method. The example below limits framework logs (category begins with "Microsoft" or "System") to warnings while logging at debug level for logs created by application code.

public void Configure(IApplicationBuilder app,
    IHostingEnvironment env,
    ILoggerFactory loggerFactory)
{
    loggerFactory
        .WithFilter(new FilterLoggerSettings
        {
            { "Microsoft", LogLevel.Warning },
            { "System", LogLevel.Warning },
            { "ToDoApi", LogLevel.Debug }
        })
        .AddConsole()
        .AddDebug();

To prevent any logs from being written, specify LogLevel.None as the minimum log level. The integer value of LogLevel.None is 6, which is higher than LogLevel.Critical (5).

The WithFilter extension method is provided by the Microsoft.Extensions.Logging.Filter NuGet package. The method returns a new ILoggerFactory instance that will filter the log messages passed to all logger providers registered with it. It doesn't affect any other ILoggerFactory instances, including the original ILoggerFactory instance.

System categories and levels

Here are some categories used by ASP.NET Core and Entity Framework Core, with notes about what logs to expect from them:

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.

Log scopes

A scope can group a set of logical operations. This grouping can be used to attach the same data to each log that's created as part of a set. For example, every log created as part of processing a transaction can include the transaction ID.

A scope is an IDisposable type that's returned by the BeginScope method and lasts until it's disposed. Use a scope by wrapping logger calls in a using block:

public IActionResult GetById(string id)
{
    TodoItem item;
    using (_logger.BeginScope("Message attached to logs created in the using block"))
    {
        _logger.LogInformation(LoggingEvents.GetItem, "Getting item {ID}", id);
        item = _todoRepository.Find(id);
        if (item == null)
        {
            _logger.LogWarning(LoggingEvents.GetItemNotFound, "GetById({ID}) NOT FOUND", id);
            return NotFound();
        }
    }
    return new ObjectResult(item);
}

The following code enables scopes for the console provider:

Program.cs:

.ConfigureLogging((hostingContext, logging) =>
{
    logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
    logging.AddConsole(options => options.IncludeScopes = true);
    logging.AddDebug();
})

Note

Configuring the IncludeScopes console logger option is required to enable scope-based logging.

For information on configuration, see the Configuration section.

Program.cs:

.ConfigureLogging((hostingContext, logging) =>
{
    logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
    logging.AddConsole(options => options.IncludeScopes = true);
    logging.AddDebug();
})

Note

Configuring the IncludeScopes console logger option is required to enable scope-based logging.

Startup.cs:

public void Configure(IApplicationBuilder app,
    IHostingEnvironment env,
    ILoggerFactory loggerFactory)
{
    loggerFactory
        .AddConsole(includeScopes: true)
        .AddDebug();

Each log message includes the scoped information:

info: TodoApi.Controllers.TodoController[1002]
      => RequestId:0HKV9C49II9CK RequestPath:/api/todo/0 => TodoApi.Controllers.TodoController.GetById (TodoApi) => Message attached to logs created in the using block
      Getting item 0
warn: TodoApi.Controllers.TodoController[4000]
      => RequestId:0HKV9C49II9CK RequestPath:/api/todo/0 => TodoApi.Controllers.TodoController.GetById (TodoApi) => Message attached to logs created in the using block
      GetById(0) NOT FOUND

Built-in logging providers

ASP.NET Core ships the following providers:

Options for Logging in Azure are covered later in this article.

For information about stdout logging, see Troubleshoot ASP.NET Core on IIS and Troubleshoot ASP.NET Core startup errors on Azure App Service.

Console provider

The Microsoft.Extensions.Logging.Console provider package sends log output to the console.

logging.AddConsole();
loggerFactory.AddConsole();

AddConsole overloads let you pass in a minimum log level, a filter function, and a boolean that indicates whether scopes are supported. Another option is to pass in an IConfiguration object, which can specify scopes support and logging levels.

The console provider has a significant impact on performance and is generally not appropriate for use in production.

When you create a new project in Visual Studio, the AddConsole method looks like this:

loggerFactory.AddConsole(Configuration.GetSection("Logging"));

This code refers to the Logging section of the appSettings.json file:

{
  "Logging": {
    "Console": {
      "IncludeScopes": false
    },
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

The settings shown limit framework logs to warnings while allowing the app to log at debug level, as explained in the Log filtering section. For more information, see Configuration.

To see console logging output, open a command prompt in the project folder and run the following command:

dotnet run

Debug provider

The Microsoft.Extensions.Logging.Debug provider package writes log output by using the System.Diagnostics.Debug class (Debug.WriteLine method calls).

On Linux, this provider writes logs to /var/log/message.

logging.AddDebug();
loggerFactory.AddDebug();

AddDebug overloads let you pass in a minimum log level or a filter function.

EventSource provider

For apps that target ASP.NET Core 1.1.0 or later, the Microsoft.Extensions.Logging.EventSource provider package can implement event tracing. On Windows, it uses ETW. The provider is cross-platform, but there are no event collection and display tools yet for Linux or macOS.

logging.AddEventSourceLogger();
loggerFactory.AddEventSourceLogger();

A good way to collect and view logs is to use the PerfView utility. There are other tools for viewing ETW logs, but PerfView provides the best experience for working with the ETW events emitted by ASP.NET.

To configure PerfView for collecting events logged by this provider, add the string *Microsoft-Extensions-Logging to the Additional Providers list. (Don't miss the asterisk at the start of the string.)

Perfview Additional Providers

Windows EventLog provider

The Microsoft.Extensions.Logging.EventLog provider package sends log output to the Windows Event Log.

logging.AddEventLog();
loggerFactory.AddEventLog();

AddEventLog overloads let you pass in EventLogSettings or a minimum log level.

TraceSource provider

The Microsoft.Extensions.Logging.TraceSource provider package uses the TraceSource libraries and providers.

logging.AddTraceSource(sourceSwitchName);
loggerFactory.AddTraceSource(sourceSwitchName);

AddTraceSource overloads let you pass in a source switch and a trace listener.

To use this provider, an app has to run on the .NET Framework (rather than .NET Core). The provider can route messages to a variety of listeners, such as the TextWriterTraceListener used in the sample app.

The following example configures a TraceSource provider that logs Warning and higher messages to the console window.

public void Configure(IApplicationBuilder app,
    IHostingEnvironment env,
    ILoggerFactory loggerFactory)
{
    loggerFactory
        .AddDebug();

    // add Trace Source logging
    var testSwitch = new SourceSwitch("sourceSwitch", "Logging Sample");
    testSwitch.Level = SourceLevels.Warning;
    loggerFactory.AddTraceSource(testSwitch,
        new TextWriterTraceListener(writer: Console.Out));

Logging in Azure

For information about logging in Azure, see the following sections:

Azure App Service provider

The Microsoft.Extensions.Logging.AzureAppServices provider package writes logs to text files in an Azure App Service app's file system and to blob storage in an Azure Storage account. The provider package is available for apps targeting .NET Core 1.1 or later.

If targeting .NET Core, note the following points:

  • Don't explicitly call AddAzureWebAppDiagnostics. The provider is automatically made available to the app when the app is deployed to Azure App Service.

If targeting .NET Framework or referencing the Microsoft.AspNetCore.App metapackage, add the provider package to the project. Invoke AddAzureWebAppDiagnostics on an ILoggerFactory instance:

logging.AddAzureWebAppDiagnostics();
loggerFactory.AddAzureWebAppDiagnostics();

An AddAzureWebAppDiagnostics overload lets you pass in AzureAppServicesDiagnosticsSettings. The settings object can override default settings, such as the logging output template, blob name, and file size limit. (Output template is a message template that's applied to all logs in addition to what's provided with an ILogger method call.)

When you deploy to an App Service app, the application honors the settings in the Diagnostic Logs section of the App Service page of the Azure portal. When these settings are updated, the changes take effect immediately without requiring a restart or redeployment of the app.

Azure logging settings

The default location for log files is in the D:\home\LogFiles\Application folder, and the default file name is diagnostics-yyyymmdd.txt. The default file size limit is 10 MB, and the default maximum number of files retained is 2. The default blob name is {app-name}{timestamp}/yyyy/mm/dd/hh/{guid}-applicationLog.txt. For more information about default behavior, see AzureAppServicesDiagnosticsSettings.

The provider only works when the project runs in the Azure environment. It has no effect when the project is run locally—it doesn't write to local files or local development storage for blobs.

Azure log streaming

Azure log streaming lets you view log activity in real time from:

  • The app server
  • The web server
  • Failed request tracing

To configure Azure log streaming:

  • Navigate to the Diagnostics Logs page from your app's portal page.
  • Set Application Logging (Filesystem) to On.

Azure portal diagnostic logs page

Navigate to the Log Streaming page to view app messages. They're logged by the app through the ILogger interface.

Azure portal application log streaming

Azure Application Insights trace logging

The Application Insights SDK can collect and report logs generated by the ASP.NET Core logging infrastructure. For more information, see the following resources:

Third-party logging providers

Third-party logging frameworks that work with ASP.NET Core:

Some third-party frameworks can perform semantic logging, also known as structured logging.

Using a third-party framework is similar to using one of the built-in providers:

  1. Add a NuGet package to your project.
  2. Call an ILoggerFactory.

For more information, see each provider's documentation. Third-party logging providers aren't supported by Microsoft.

Additional resources