How do I write logs from within Startup.cs?

.Net Core 3.1

Unfortunately, for ASP.NET Core 3.0, the situation is again a bit different. The default templates use the HostBuilder (instead of the WebHostBuilder) which sets up a new generic host that can host several different applications, not limited to web applications. Part of this new host is also the removal of the second dependency injection container that previously existed for the web host. This ultimately means that you won’t be able to inject any dependencies apart from the IConfiguration into the Startup class. So you won’t be able to log during the ConfigureServices method. You can, however, inject the logger into the Configure method and log there:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILogger<Startup> logger)
{
    logger.LogInformation("Configure called");

    // …
}

If you absolutely need to log within ConfigureServices, then you can continue to use the WebHostBuilder which will create the legacy WebHost that can inject the logger into the Startup class. Note that it’s likely that the web host will be removed at some point in the future. So you should try to find a solution that works for you without having to log within ConfigureServices.

.NET Core 2.x

This has changed significantly with the release of ASP.NET Core 2.0. In ASP.NET Core 2.x, logging is created at the host builder. This means that logging is available through DI by default and can be injected into the Startup class:

public class Startup
{
    private readonly ILogger<Startup> _logger;

    public IConfiguration Configuration { get; }

    public Startup(ILogger<Startup> logger, IConfiguration configuration)
    {
        _logger = logger;
        Configuration = configuration;
    }

    public void ConfigureServices(IServiceCollection services)
    {
        _logger.LogInformation("ConfigureServices called");

        // …
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        _logger.LogInformation("Configure called");

        // …
    }
}

Option 1: Directly use log (e.g. Serilog) in startup-

public class Startup
{
    public Startup(IHostingEnvironment env)
    {
        Log.Logger = new LoggerConfiguration()
           .MinimumLevel.Debug()
           .WriteTo.RollingFile(Path.Combine(env.ContentRootPath, "Serilog-{Date}.txt"))
           .CreateLogger();

        Log.Information("Inside Startup ctor");
        ....
    }

    public void ConfigureServices(IServiceCollection services)
    {
        Log.Information("ConfigureServices");
        ....
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        Log.Information("Configure");
        ....
    }

Output:

https://i.stack.imgur.com/96E8P.png

To setup Serilog in asp.net-core application, check out the Serilog.AspNetCore package on GitHub.

Option2: Configure logging in program.cs like this-

var host = new WebHostBuilder()
            .UseKestrel()
            .ConfigureServices(s => {
                s.AddSingleton<IFormatter, LowercaseFormatter>();
            })
            .ConfigureLogging(f => f.AddConsole(LogLevel.Debug))
            .UseStartup<Startup>()
            .Build();

host.Run();

User loggerFactory in startup like this-

public class Startup
{
    ILogger _logger;
    IFormatter _formatter;
    public Startup(ILoggerFactory loggerFactory, IFormatter formatter)
    {
        _logger = loggerFactory.CreateLogger<Startup>();
        _formatter = formatter;
    }

    public void ConfigureServices(IServiceCollection services)
    {
        _logger.LogDebug($"Total Services Initially: {services.Count}");

        // register services
        //services.AddSingleton<IFoo, Foo>();
    }

    public void Configure(IApplicationBuilder app, IFormatter formatter)
    {
        // note: can request IFormatter here as well as via constructor
        _logger.LogDebug("Configure() started...");
        app.Run(async (context) => await context.Response.WriteAsync(_formatter.Format("Hi!")));
        _logger.LogDebug("Configure() complete.");
    }
}

Complete details available on this link

In .NET Core 3.1, you can create a logger directly using LogFactory.

var loggerFactory = LoggerFactory.Create(builder =>
{
     builder.AddConsole();                
});

ILogger logger = loggerFactory.CreateLogger<Startup>();
logger.LogInformation("Example log message");

The official solution is currently to setup a local LoggerFactory like this:

    using var loggerFactory = LoggerFactory.Create(builder =>
    {
        builder.SetMinimumLevel(LogLevel.Information);
        builder.AddConsole();
        builder.AddEventSourceLogger();
    });
    var logger = loggerFactory.CreateLogger("Startup");
    logger.LogInformation("Hello World");

See also: https://github.com/dotnet/aspnetcore/issues/9337#issuecomment-539859667

I use a solution avoiding 3rd party loggers implementing a "logger buffer" with ILogger interface.

public class LoggerBuffered : ILogger
{
    class Entry
    {
        public LogLevel _logLevel;
        public EventId  _eventId;
        public string   _message;
    }
    LogLevel            _minLogLevel;
    List<Entry>         _buffer;
    public LoggerBuffered(LogLevel minLogLevel)
    {
        _minLogLevel = minLogLevel;
        _buffer = new List<Entry>();
    }
    public IDisposable BeginScope<TState>(TState state)
    {
        return null;
    }

    public bool IsEnabled(LogLevel logLevel)
    {
        return logLevel >= _minLogLevel;
    }

    public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
    {
        if (IsEnabled(logLevel)) {
            var str = formatter(state, exception);
            _buffer.Add(new Entry { _logLevel = logLevel, _eventId = eventId, _message = str });
        }
    }
    public void CopyToLogger (ILogger logger)
    {
        foreach (var entry in _buffer)
        {
            logger.Log(entry._logLevel, entry._eventId, entry._message);
        }
        _buffer.Clear();
    }
}

Usage in startup.cs is easy, of course you get log output after call of Configure. But better than nothing. :

public class Startup
{
ILogger         _logger;

public Startup(IConfiguration configuration, IWebHostEnvironment env)
{
    _logger = new LoggerBuffered(LogLevel.Debug);
    _logger.LogInformation($"Create Startup {env.ApplicationName} - {env.EnvironmentName}");

}

public void ConfigureServices(IServiceCollection services)
{
    _logger.LogInformation("ConfigureServices");
    services.AddControllersWithViews();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, ILogger<Startup> logger)
{
    (_logger as LoggerBuffered).CopyToLogger(logger);
    _logger = logger;   // Replace buffered by "real" logger
    _logger.LogInformation("Configure");

    if (env.IsDevelopment())

For .NET Core 3.0 the official docs has this to say: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/logging/?view=aspnetcore-3.0#create-logs-in-startup

Writing logs before completion of the DI container setup in the Startup.ConfigureServices method is not supported: Logger injection into the Startup constructor is not supported. Logger injection into the Startup.ConfigureServices method signature is not supported

But as they say in the docs you can configure a service that depends on ILogger, so if you wrote a class StartupLogger:

public class StartupLogger
{
    private readonly ILogger _logger;

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

    public void Log(string message)
    {
        _logger.LogInformation(message);
    }
}

Then in Startup.ConfigureServices add the service, then you need to build the service provider to get access to the DI container:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton(provider =>
    {
        var service = provider.GetRequiredService<ILogger<StartupLogger>>();
        return new StartupLogger(service);
    });
    var logger = services.BuildServiceProvider().GetRequiredService<StartupLogger>();
    logger.Log("Startup.ConfigureServices called");
}

Edit: this produces a compiler warning, for the sake of debugging your StartUp class this should be OK but not for production:

Startup.cs(39, 32): [ASP0000] Calling 'BuildServiceProvider' from application code results in an additional copy of singleton services being created. Consider alternatives such as dependency injecting services as parameters to 'Configure'.

None of the existing answers worked for me. I'm using NLog, and even building a new ServiceCollection, calling .CreateBuilder() on any service collection, creating a logging service ... none of that would write to a log file during ConfigureServices.

The problem is that logging isn't really a thing until after the ServiceCollection is built, and it's not built during ConfigureServices.

Basically, I just want (need) to log what's going on during startup in a configuration extension method, because the only tier I'm having a problem on is PROD, where I can't attach a debugger.

The solution that worked for me was using the old .NET Framework NLog method:

private static readonly NLog.Logger Logger = NLog.LogManager.GetCurrentClassLogger();

Added that right to the extension method class, and I was able to write to a log ("the" log) during ConfigureServices and after.

I have no idea if this is a good idea to actually release into production code (I don't know if the .NET controlled ILogger and this NLog.ILogger will conflict at any point), but I only needed it to see what was going on.

Using Rolf's answer, I put this in my Startup constructor:

private readonly ILogger _logger;

public Startup(IConfiguration configuration)
{
    Configuration = configuration;

    using var loggerFactory = LoggerFactory.Create(builder =>
    {
        builder.SetMinimumLevel(LogLevel.Information);
        builder.AddConsole();
        builder.AddEventSourceLogger();
    });
    _logger = loggerFactory.CreateLogger<Startup>();
}

public void ConfigureServices(IServiceCollection services)
{
    _logger.LogInformation("ConfigureServices...");
    // ...and so on...
}

Main code:

public class Program
{
    public static void Main(string[] args)
    {
        BuildWebHost(args).Run();
    }

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

CreateDefaultBuilder sets up a default console logger.

... configures the ILoggerFactory to log to the console and debug output

Startup code:

using Microsoft.Extensions.Logging;
...
public class Startup
{
    private readonly ILogger _logger;

    public Startup(IConfiguration configuration, ILoggerFactory logFactory)
    {
        _logger = logFactory.CreateLogger<Startup>();
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        _logger.LogInformation("hello stackoverflow");
    }

I couldn't get the injection of an ILogger to work, but perhaps that's because it's not a controller. More info welcome!

Refs:

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/logging/?view=aspnetcore-2.1&tabs=aspnetcore2x

For .Net 6

var builder = WebApplication.CreateBuilder(args);
...
var app = builder.Build();
var logger = ((IApplicationBuilder)app).ApplicationServices.GetService<ILogger<Program>>();
logger.LogInformation("Some logs");

Or even more easy way:

var builder = WebApplication.CreateBuilder(args);
...
var app = builder.Build();
ILogger logger = app.Logger;

Are you making decisions about which services you are using at runtime that you wish to log? Or are you making decisions about how those services are configured, which you wish to log?

In other words;

public void ConfigureServices(IServiceCollection services){
   // Do you really want to log something here?
   services.AddRazorPages(options => {
       // Or would you be satisfied by logging something here?
   });
}

If it is only the latter, you can move the implementation of these lambda functions into an IConfigureOptions<T> service, allowing you to inject other services. Continuing the above example, you could create the following;

public class ConfigureRazorPagesOptions : IConfigureOptions<RazorPagesOptions>
{
    private readonly ILogger<ConfigureRazorPagesOptions> logger;
    public ConfigureRazorPagesOptions(ILogger<ConfigureRazorPagesOptions> logger)
    {
        this.logger = logger;
    }

    public void Configure(RazorPagesOptions options)
    {
        logger.LogInformation("Now I can log here!");
    }
}

public void ConfigureServices(IServiceCollection services){
   services.AddRazorPages();
   services.AddSingleton<IConfigureOptions<RazorPagesOptions>, ConfigureRazorPagesOptions>();
}

If your .ConfigureServices method is getting too complicated, you might want to create such services. However, that's a lot of boilerplate to add for each options type. There is also an equivalent shorthand, to inject other services into a configuration lamda;

services.AddOptions<RazorPagesOptions>()
    .Configure<ILogger<RazorPagesOptions>>((options, logger) => { 
        logger.LogInformation("I can log here too!");
    });

This worked for me

private static readonly Logger logger = LogManager.GetLogger("Audit")

I found a very easy implementation:

public void ConfigureServices(IServiceCollection services)
{
     services.AddControllersWithViews();

     var conn = Configuration.GetValue("conn", Configuration.GetConnectionString("Conn"));

     Console.WriteLine($@"System starting at {DateTime.Now}");

     Console.WriteLine($@"Database: {conn}");
}

Just using Console.WriteLine worked, even on Docker.

Just use the line below for logging in Startup.cs

Log.Information("App started.");