Unable to resolve ILogger from Microsoft.Extension

问题:

I've configured my console application's Main like so

var services = new ServiceCollection()
                .AddLogging(logging => logging.AddConsole())
                .BuildServiceProvider();

And then I try to use it in another class like so

    private readonly ILogger _logger;

    public MyClass(ILogger logger)
    {
        _logger = logger;
    }

    public void MyFunc()
    {
        _logger.Log(LogLevel.Error, "My Message");
    }

System.InvalidOperationException: 'Unable to resolve service for type 'Microsoft.Extensions.Logging.ILogger'

I've tried the solutions here but it didn't work for me.

Edit Based on Yaakov's comment below and this Github comment I'm able to resolve it correctly by doing this

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

I would have preferred to have this in the initial BuildServiceProvider but looks like I'm gonna have to repeat this every time I want to use the logger (or create my own ILogger).

回答1:

ILogger is no longer registered by default but ILogger<T> is. If you still want to use ILogger you can registered it manually with the following (Startup.cs):

    public void ConfigureServices(IServiceCollection services)
    {
        var serviceProvider = services.BuildServiceProvider();
        var logger = serviceProvider.GetService<ILogger<AnyClass>>();
        services.AddSingleton(typeof(ILogger), logger);
        ...
     }

Where AnyClass can be something generic, such as:

     public class ApplicationLogs
     {
     }

So:

        public void ConfigureServices(IServiceCollection services)
        {
            var serviceProvider = services.BuildServiceProvider();
            var logger = serviceProvider.GetService<ILogger<ApplicationLog>>();
            services.AddSingleton(typeof(ILogger), logger);
            ...
         }

ILogger will now resolve via constructor injection

回答2:

I am assuming you are using the default template for .net core web application.
In your your Startup.cs you should have a method like this↓↓↓↓↓↓↓

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
        {
            // This lambda determines whether user consent for non-essential cookies is needed for a given request.
            options.CheckConsentNeeded = context => true;
            options.MinimumSameSitePolicy = SameSiteMode.None;
        });


        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
        //Do the service register here and extra stuff you want
         services.AddLogging(config =>
        {
            config.AddDebug();
            config.AddConsole();
            //etc
        });

    }

Edit: I have written a simple program for you to show how it works

public class MyClass
{

    private readonly ILogger<MyClass> _logger;


    public MyClass(ILogger<MyClass> logger)
    {
        _logger = logger;
    }
    public void MyFunc()
    {
        _logger.Log(LogLevel.Error, "My Message");
    }
}



public class Program
{
    public static void Main(string[] args)
    {
        var services = new ServiceCollection().AddLogging(logging => logging.AddConsole());
        services.AddSingleton<MyClass>();//Singleton or transient?!
        var s = services.BuildServiceProvider();
        var myclass = s.GetService<MyClass>();
     }
}

Edit: Output of the program: