Does injecting ILogger<T> create a new logger each time?

Question

On the logging samples in the documentation, there is an example how to inject a logger into a controller:

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

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

Does the DI framework create a new logger each time I inject a logger into something like here? Is there a better way?

Answer 1

This is easily answered by a look into the source. When you do services.AddLogging(), the default behavior is that ILogger<T> is registered as a singleton:

public static IServiceCollection AddLogging(this IServiceCollection services, Action<ILoggingBuilder> configure)
{
    // …

    services.TryAdd(ServiceDescriptor.Singleton<ILoggerFactory, LoggerFactory>());
    services.TryAdd(ServiceDescriptor.Singleton(typeof(ILogger<>), typeof(Logger<>)));

    // …
}

So no, ILogger<T> instances for a certain type T are kept around for as long as the application is running. So when injecting an ILogger<TodoController> into your controller, the same logger instance will be passed to it every time.

Of course this only applies to the logger, but not your controller itself. By default, controllers are activated outside of DI but effectively live with a scoped lifetime. So on every request, there will be a new controller instance; but that one will then get the logger instance from before.

To answer your last question, is there a better way? No. Apart from the fact that this behavior is already a good one (since there’s no need for new logger instances), the proper way to use logging is indeed to inject ILogger<T> into types T, so you get a properly categorized logger instance. There’s really no need to worry about the very thin logger here when there are so many way more expensive things going on in the background that you will likely never see ;)

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Since the ILogger<T> is a singleton, its instance will be reused all throughout the application. Note that this will not have an effect on logging scopes. The ILogger<T> implementation that you use within your application is actually just a thin wrapper that forwards logging calls to the internal loggers (which are also effectively singletons). So the lifetime of ILogger<T> is actually not relevant since they do not keep any state at all.

The logging scopes themselves are persisted using an AsyncLocal which is a mechanism to keep state throughout the asynchronous call flow. That means that logging scopes will just “magically” work and not leak outside of the call flow just because some instances happen to be shared between multiple threads (or asynchronous flows).