ASP.NET Core - Logging

ASP.NET Core framework provides built-in supports for logging. However, we can also use third party logging provider easily in ASP.NET Core application.

Before we see how to implement logging in ASP.NET Core application, let's understand the important logging interfaces and classes available in ASP.NET Core framework. The following are built-in interfaces and classes available for logging under Microsoft.Extensions.Logging namespace .

1. ILoggingFactory
2. ILoggingProvider
3. ILogger
4. LoggingFactory

The following figure shows the relationship between logging classes.

Logging Infrastructure

Let's have an overview on each of them.

ILoggerFactory

The ILoggerFactory is the factory interface for creating an appropriate ILogger type instance and also to add ILoggerProvider instance.

ILoggerFactory:

public interface ILoggerFactory : IDisposable
{
    ILogger CreateLogger(string categoryName);
    void AddProvider(ILoggerProvider provider);
}

ASP.NET Core framework includes built-in LoggerFactory class that implements ILoggerFactoryinterface. We can use it to add an instance of type ILoggerProvider and to retrieve ILoggerinstance for the specified category.

ASP.NET Core runtime creates an instance of LoggerFactory class and registers it for ILoggerFactory with the built-in IoC container when the application starts. Thus, we can use ILoggerFactory interface anywhere in your application. The IoC container will pass an instance of LoggerFactory to your application wherever it encounters ILoggerFactory type.

ILoggerProvider

The ILoggerProvider manages and creates appropriate logger specified by logging category.

ILoggerProvider:

public interface ILoggerProvider : IDisposable
{
   ILogger CreateLogger(string categoryName);
}

We can create our own logging provider by implementing ILoggerProvider interface.

ILogger

ILogger interface includes methods for logging to the underlying storage.

ILogger:

public interface ILogger
{
    void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter);
    bool IsEnabled(LogLevel logLevel);
    IDisposable BeginScope<TState>(TState state);
} 

Built-in Logging Providers

There are different logging providers available as NuGet packages which we can use to send log output to the different medium such as console, debug window, EventSource etc. ASP.NET Core ships with the following providers:

1. Console
2. Debug
3. EventSource
4. EventLog
5. TraceSource
6. Azure App Service

Let's have an overview of Console logger.

Console Logger

The Microsoft.Extensions.Logging.Console package includes logging classes which sends log output to the console. The following are important classes for console logging.

1. ConsoleLoggingProvider
2. ConsoleLogger
3. ConsoleLoggerExtension
4. ConsoleLoggerSettings
5. ConsoleLoggerScope

The following figure illustrates the console logger classes.

Logging Infrastructure

As you can see in the above figure, the ConsoleLogger implements ILogger and ConsoleLoggingProvider implements ILoggingProvider. The ConsoleLoggerExtensions class includes extension method AddConsole() which adds console logger to LoggerFactory.

Now, let's use console logger to display log output to the console in both ASP.NET Core 1.x and 2.x application.

Step 1 - Install NuGet Package

ASP.NET Core 1.x:

Include Microsoft.Extensions.Logging.Console dependencies in project.json if it is not added already.

"dependencies": {
        "Microsoft.NETCore.App": {
            "version": "1.0.1",
            "type": "platform"
        },
        "Microsoft.AspNetCore.Diagnostics": "1.0.0",
        "Microsoft.AspNetCore.Server.IISIntegration": "1.0.0",
        "Microsoft.AspNetCore.Server.Kestrel": "1.0.1",
        "Microsoft.Extensions.Logging.Console": "1.0.0"
    } 

Visual Studio will restore the packages automatically as soon as project.json is saved.

ASP.NET Core 2.x:

By default, Microsoft.Extensions.Logging.Console is included in the meta package Microsoft.AspNetCore.All, so we don't need to install it separately in ASP.NET Core 2.x application.

Step 2 - Use Provider

ASP.NET Core 1.x:

Now, in order to use console logger, we first need to configure it. First, we need to add ConsoleLoggerProvider to the providers list using LoggerFactory. As you can see in the LoggerFactory class above, it provides AddProvider() method which adds our custom ILoggerProvider type instance to the list. The Microsoft.Extensions.Logging.Console package includes all necessary classes. So, we can add ConsoleLoggerProvider instance as shown below.

Example: Add ConsoleLoggerProvider

public class Startup
{
    public Startup()
    {
    } 
 
    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        loggerFactory.AddProvider(new ConsoleLoggerProvider((category, logLevel) => logLevel >= LogLevel.Information, false));
        
        //code removed for clarity 
    }
}

As you can see above, we add an instance of ConsoleLoggerProvider to the factory. Now, we can start logging. The console will display all the logs whose LogLevel is Information or above.

We can also use AddConsole() extension method instead of configuring console logger manually as shown below.

Example: Add Console Logger

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

        //code removed for clarity 
    }
}

ASP.NET Core 2.x:

The CreateDefaultBuilder() already includes console and debug logging providers. So there is no need to add it again in the Configure() method in ASP.NET Core 2.x application.

Step 3 - Create Logs

This step is applicable for both ASP.NET Core 1.x and 2.x application.

Now, we can create logs and see it on the console by getting an instance of ILogger using LoggerFactory and start logging as shown below.

Example: Add Console Logger

public class Startup
{
    public Startup()
    {
    } 
 
    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {

        loggerFactory.AddConsole();

        //start logging to the console
        var logger = loggerFactory.CreateLogger<ConsoleLogger>();
        logger.LogInformation("Executing Configure()");

        //code removed for clarity 
    }
}

We can also use ILogger anywhere in our application. IoC container will inject ConsoleLoggerinstance wherever it sees ILogger. For example, we can use ILogger in the MVC-controller as shown below.

Example: Logging in MVC Controller

public class HomeController : Controller
{
    ILogger _logger;

    public HomeController(ILogger<HomeController> logger)
    {
        _logger = logger;
    }
    public IActionResult Index()
    {
        _logger.LogInformation("Executing Home/Index");

        return View();
    } 
}

You will see the above log on the console when you browse http://localhost:5000/home/index.

We can use any of the above mentioned built-in logging providers by following the same process.

Log Levels

Log levels indicate the importance or severity of the log messages. Built-in log providers include extension methods to indicate log levels.

ASP.NET Core defines following log levels.

Log Level	Severity	Extension Method	Description
Trace	0	LogTrace()	Log messages only for tracing purpose for the developers
Debug	1	LogDebug()	Log messages for short-term debugging purpose
Information	2	LogInformation()	Log messages for the flow of the application.
Warning	3	LogWarning()	Log messages for abnormal or unexpected events in the application flow.
Error	4	LogError()	Log error messages.
Critical	5	LogCritical()	Log messages for the failures that require immediate attention

We can use extension method to indicate level of the log messages as shown below.

Example: Log Level

public class HomeController : Controller
{
    ILogger _logger;

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

    public IActionResult Index(string id)
    {
        _logger.LogInformation("Home/Index Executing..");

        if (String. IsNullOrEmpty(id))
        {
            _logger.LogWarning(LoggingEvents.GET_ITEM_NOTFOUND, "Index({ID}) NOT FOUND", id);

            return NotFound();
        }

        return View();
    } 
}

Third-party Logging Providers

The following are some logging providers that work with ASP.NET Core:

Logging Provider	Description
elmah.io	Provider for the Elmah.Io service
Loggr	Provider for the Logger service
NLog	Provider for the NLog library
Serilog	Provider for the Serilog library

Learn about ASP.NET Core environment variables in the next chapter.