In order to debug a .NET Core app which is failing on startup, I would like to write logs from within the startup.cs file. I have logging setup within the file that can be used in the rest of the app outside the startup.cs file, but not sure how to write logs from within the startup.cs file itself.
166
votes
12 Answers
235
votes
.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");
// …
}
}
46
votes
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:
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
17
votes
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");
11
votes
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
5
votes
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())
5
votes
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.ConfigureServicesmethod is not supported:
- Logger injection into the
Startupconstructor is not supported.- Logger injection into the
Startup.ConfigureServicesmethod 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'.
5
votes
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.
2
votes
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:
1
votes
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...
}
0
votes
