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

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.ConfigureServices method is not supported:

• Logger injection into the Startup constructor is not supported.
• Logger injection into the Startup.ConfigureServices method 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 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>();
        return new StartupLogger(service);
    });
    var logger = services.BuildServiceProvider().GetRequiredService();
    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'.