Should I take ILogger, ILogger, ILoggerFactory or ILoggerProvider for a library?

Definition

We have 3 interfaces: ILogger, ILoggerProvider and ILoggerFactory. Let’s look at the source code to find out their responsibilities:

ILogger: is responsible to write a log message of a given Log Level.

ILoggerProvider: is responsible to create an instance of ILogger (you are not supposed to use ILoggerProvider directly to create a logger)

ILoggerFactory: you can register one or more ILoggerProviders with the factory, which in turn uses all of them to create an instance of ILogger. ILoggerFactory holds a collection of ILoggerProviders.

In the example below, we are registering 2 providers (console and file) with the factory. When we create a logger, the factory uses both of these providers to create an instance of Logger:

ILoggerFactory factory = new LoggerFactory().AddConsole();    // add console provider
factory.AddProvider(new LoggerFileProvider("c:\\log.txt"));   // add file provider
Logger logger = factory.CreateLogger(); // creates a console logger and a file logger

So the logger itself, is maintaining a collection of ILoggers, and it writes the log message to all of them. Looking at Logger source code we can confirm that Logger has an array of ILoggers (i.e. LoggerInformation[]), and at the same time it is implementing ILogger interface.

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Dependency Injection

MS documentation provides 2 methods for injecting a logger:

1. Injecting the factory:

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

creates a Logger with Category = TodoApi.Controllers.TodoController.

2. Injecting a generic ILogger<T>:

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

creates a logger with Category = fully qualified type name of TodoController

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In my opinion, what makes the documentation confusing is that it does not mention anything about injecting a non-generic, ILogger. In the same example above, we are injecting a non-generic ITodoRepository and yet, it does not explain why we are not doing the same for ILogger.

According to Mark Seemann:

An Injection Constructor should do no more than receiving the
dependencies.

Injecting a factory into the Controller is not a good approach, because it is not Controller’s responsibility to initialize the Logger (violation of SRP). At the same time injecting a generic ILogger<T> adds unnecessary noise. See Simple Injector’s blog for more details: What’s wrong with the ASP.NET Core DI abstraction?

What should be injected (at least according to the article above) is a non-generic ILogger, but then, that’s not something that Microsoft’s Built-in DI Container can do, and you need to use a 3rd party DI Library. These two documents explain how you can use 3rd party libraries with .NET Core.

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This is another article by Nikola Malovic, in which he explains his 5 laws of IoC.

Nikola’s 4th law of IoC

Every constructor of a class being resolved should not have any
implementation other than accepting a set of its own dependencies.