How can I inject dependencies into a custom ILogger in asp.net core 2.0?

Question

In asp.net core 1.1 I could inject the IServiceProvider into the logger provider and resolve my logger when CreateLogger was called, but it all changed in asp.net core 2.0

My ILogger implementation needs dependencies injected. How can I achieve this?

Answer 1

ASP.NET core provides possibility to replace built-in DI container with custom one (see this article for details). You could use this possibility to obtain instance of IServiceProvider earlier for logging bootstrapping while still using standard .Net core DI container.

To do this you should change return value of Startup.ConfigureServices(IServiceCollection services) method from void to IServiceProvider. You can use this possibility to build instance of IServiceProvider in ConfigureServices, use it for logging bootstrapping and then return from the method.

Sample code:

public interface ISomeDependency
{
}

public class SomeDependency : ISomeDependency
{
}

public class CustomLogger : ILogger
{
    public CustomLogger(ISomeDependency dependency)
    {
    }

    //  ...
}

public class CustomLoggerProvider : ILoggerProvider
{
    private readonly IServiceProvider serviceProvider;

    public CustomLoggerProvider(IServiceProvider serviceProvider)
    {
        this.serviceProvider = serviceProvider;
    }

    public ILogger CreateLogger(string categoryName)
    {
        return serviceProvider.GetRequiredService<ILogger>();
    }

    //  ...
}

public class Startup
{
    // This method gets called by the runtime. Use this method to add services to the container.
    public IServiceProvider ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();

        return ConfigureLogging(services);
    }

    private IServiceProvider ConfigureLogging(IServiceCollection services)
    {
        services.AddTransient<ISomeDependency, SomeDependency>();
        services.AddSingleton<ILogger, CustomLogger>();
        IServiceProvider serviceProvider = services.BuildServiceProvider();

        var loggerFactory = new LoggerFactory();
        loggerFactory.AddProvider(new CustomLoggerProvider(serviceProvider));

        return serviceProvider;
    }

    //  ...
}

Answer 2

If you are configuring logging in program.cs you can create a function to configure logging and get an instance of logging provider like this:

private static void ConfigureApplicationLogging(WebHostBuilderContext context, ILoggingBuilder loggingBuilder)
    {
        loggingBuilder.AddConfiguration(context.Configuration.GetSection("Logging"));
        loggingBuilder.AddDebug();
        loggingBuilder.AddConsole();

        var serviceProvider = loggingBuilder.Services.BuildServiceProvider();
        loggingBuilder.AddProvider(new DoxErrorLoggerProvider(serviceProvider, null));
    }

Then in BuildWebHost you will configure logging as follows:

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