How do I write logs from within Startup.cs?

Question

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 u

Answer 1

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         _buffer;
    public LoggerBuffered(LogLevel minLogLevel)
    {
        _minLogLevel = minLogLevel;
        _buffer = new List();
    }
    public IDisposable BeginScope(TState state)
    {
        return null;
    }

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

    public void Log(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func 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 logger)
{
    (_logger as LoggerBuffered).CopyToLogger(logger);
    _logger = logger;   // Replace buffered by "real" logger
    _logger.LogInformation("Configure");

    if (env.IsDevelopment())