How to log to a file without using third party logger in .Net Core?

Issue http://github.com/aspnet/Logging/issues/441 is closed and MS officially recommends to use 3rd party file loggers. You might want to avoid using heavyweight logging frameworks like serilog, nlog etc because they are just excessive in case if all you need is a simple logger that writes to a file and nothing more (without any additional dependencies).

I faced the same situation, and implemented simple (but efficient) file logger: https://github.com/nreco/logging

• can be used in .NET Core 1.x and .NET Core 2.x / 3.x / 4.x / 5.x and .NET 6.0 LTS apps
• supports custom log message handler for writing logs in JSON or CSV
• implements simple 'rolling file' feature if max log file size is specified
• Support daily file format log (eg. 2022-03-31.log, 2022-03-30.log)

.NET Core does not (and probably will not) provide a built-in ILoggerProvider implementation for file logging.

There is a facade which makes trace source logging (the built-in logger framework originated in classic .NET) available for .NET Core applications. It can be ok for those who are already familiar with it, but be prepared that configuration is somewhat cumbersome on .NET Core (for details, see this nice article).

As an alternative, you may try my lightweight ILogger<T> implementation which covers the features of the built-in ConsoleLogger and provides additional essential features and good customizability. My library is free, open-source and has only framework dependencies. It completely conforms with the Microsoft provider implementations.

Usage is as simple as follows:

dotnet add package Karambolo.Extensions.Logging.File

ASP.NET Core 6+ web applications (minimal hosting model):

var builder = WebApplication.CreateBuilder(args);

builder.Logging.AddFile(o => o.RootPath = o.RootPath = builder.Environment.ContentRootPath);

var app = builder.Build();

// ...

ASP.NET Core 3.1+ web applications:

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder
                .ConfigureLogging((ctx, builder) =>
                {
                    builder.AddConfiguration(ctx.Configuration.GetSection("Logging"));
                    builder.AddFile(o => o.RootPath = ctx.HostingEnvironment.ContentRootPath);
                })
                .UseStartup<Startup>();
        });

.NET Core 3.1/.NET 5+ console applications:

// build configuration
// var configuration = ...;

// configure DI
var services = new ServiceCollection();

services.AddLogging(builder =>
{
    builder.AddConfiguration(configuration.GetSection("Logging"));
    builder.AddFile(o => o.RootPath = AppContext.BaseDirectory);
});

// create logger factory
await using (var sp = services.BuildServiceProvider())
{
    var loggerFactory = sp.GetService<ILoggerFactory>();
    // ...
}

As of v3.3.0 structured logging is also supported. JSON format is available out of the box, as a separate package:

dotnet add package Karambolo.Extensions.Logging.File.Json

For configuration details, see the project site.

The ones provided by Adam and Vitaliy are still probably the most simple ones to date (thanks guys btw!).
Since an external NuGet is necessary anyway, worth mentioning that there is also a "standalone" extension of the Serilog rolling file sink that can be simply used as a logging provider for .net core, without completely swapping out the logging pipeline (pulls other dependencies, but I don't see that as an issue if you need the few extra features provided)

As of March 2020, this is the complete picture:

• Karambolo.Extensions.Logging.File
• NReco.Logging.File
• Serilog.Extensions.Logging.File