Para implementar registro de logs em uma aplicação .NET Core com Log4Net, siga estas etapas essenciais.

Crie um projeto .NET Core padrão e adicione os pacotes NuGet log4net e Microsoft.Extensions.Logging.Log4Net.AspNetCore via gerenciador de pacotes.

Configure o arquivo log4net.config no diretório do projeto com estrutura específica para difreentes níveis de log. Exemplo:

<?xml version="1.0" encoding="utf-8" ?>
<log4net>
  <root>
    <level value="All" />
    <appender-ref ref="CriticalAppender" />
    <appender-ref ref="WarningAppender" />
    <appender-ref ref="StandardAppender" />
  </root>

  <appender name="CriticalAppender" type="log4net.Appender.RollingFileAppender">
    <file value="Logs/Critical/" />
    <appendToFile value="true" />
    <rollingStyle value="Date" />
    <datePattern value="yyyy-MM-dd_critical.log" />
    <maxSizeRollBackups value="50" />
    <staticLogFileName value="false" />
    <encoding value="utf-8" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %level - %message%newline" />
    </layout>
    <filter type="log4net.Filter.LevelRangeFilter">
      <levelMin value="ERROR" />
      <levelMax value="FATAL" />
    </filter>
  </appender>

  <appender name="WarningAppender" type="log4net.Appender.RollingFileAppender">
    <file value="Logs/Warnings/" />
    <appendToFile value="true" />
    <rollingStyle value="Date" />
    <datePattern value="yyyy-MM-dd_warning.log" />
    <maxSizeRollBackups value="50" />
    <staticLogFileName value="false" />
    <encoding value="utf-8" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %level - %message%newline" />
    </layout>
    <filter type="log4net.Filter.LevelRangeFilter">
      <levelMin value="WARN" />
      <levelMax value="WARN" />
    </filter>
  </appender>

  <appender name="StandardAppender" type="log4net.Appender.RollingFileAppender">
    <file value="Logs/Standard/" />
    <appendToFile value="true" />
    <rollingStyle value="Date" />
    <datePattern value="yyyy-MM-dd_standard.log" />
    <maxSizeRollBackups value="50" />
    <staticLogFileName value="false" />
    <encoding value="utf-8" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %level - %message%newline" />
    </layout>
    <filter type="log4net.Filter.LevelRangeFilter">
      <levelMin value="INFO" />
      <levelMax value="INFO" />
    </filter>
  </appender>
</log4net>

No arquivo Program.cs, integre o Log4Net configurando o builder de logging:

public static IHostBuilder BuildHost(string[] commandArgs) =>
    Host.CreateDefaultBuilder(commandArgs)
        .ConfigureLogging(builder =>
        {
            builder.AddLog4Net();
        })
        .ConfigureWebHostDefaults(host =>
        {
            host.UseStartup<ApplicationStartup>();
        });

Para registrar logs em classes específicas, injete ILoggerFactory e crie instâncias:

private readonly ILogger _logger;

public HomeController(ILoggerFactory factory)
{
    _logger = factory.CreateLogger<HomeController>();
}

Utilize métodos como LogInformation para registrar mensagens:

_logger.LogInformation("Processo de autenticação concluído com sucesso.");