How to write logs from Startup.cs

Question

To debug the main .net application, which does not work at startup, I would like to write logs from the startup.cs file. I have a logging setting in a file that can be used in the rest of the application outside the startup.cs file, but am not sure how to write the logs from the startup.cs file itself.

Answer 1

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

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");

        // …
    }
}

---

, ASP.NET Core 3.0 . HostBuilder ( WebHostBuilder), , , -. , -. , , IConfiguration Startup. , ConfigureServices. Configure :

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

    // …
}

ConfigureServices, WebHostBuilder WebHost Startup. , , - . , , ConfigureServices.

Answer 2

1: (, Serilog) -

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");
        ....
    }

:

Serilog asp.net-core, Serilog.AspNetCore GitHub.

---

2: program.cs -

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

host.Run();

loggerFactory , -

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.");
    }
}

Answer 3

, Nlog, .

private readonly NLog.Logger _logger = new NLog.LogFactory().GetCurrentClassLogger();

Answer 4

:

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 .

... ILoggerFactory

:

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");
    }

ILogger, , , , . !

Refs:

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