.net core 3.1 WorkerService not loading configuration options/settings

Question

Using the stock project template for WorkerService (.net core 3.1) I can't get IOptions<x> to work.

Here's a minimal reproduction:

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .UseConsoleLifetime()
            .ConfigureServices((hostContext, services) =>
            {
                services.AddOptions();
                services.AddHostedService<Worker>();
            });
}

mailsettings

public class mailsettings
{
    public string smtpserver { get; set; }
}

Worker

public class Worker : BackgroundService
{
    private readonly mailsettings _mailsettings;

    public Worker(IOptions<mailsettings> mailsettings)
    {
        _mailsettings = mailsettings.Value;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            Console.WriteLine($"MailSettings SmtpServer: \"{(_mailsettings.smtpserver ?? "**null**")}\"");
            Console.WriteLine(Directory.GetCurrentDirectory());
            await Task.Delay(1000, stoppingToken);
        }
    }
}

appsettings.json and appsettings.development.json content follows:

{
  "mailsettings": {
    "smtpserver": "mail.com"
  }
}

This will write:

MailSettings SmtpServer: "null"

to the console. It should write

"mail.com".

I've tried a ton a guesses... but can't get anywhere.

Answer 1

The mail settings was not configured into the service collection, so the DI container is unable to resolve IOPtions<mailsettings>

Call Configure<TOPtions> on the service collection

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .UseConsoleLifetime()
        .ConfigureServices((hostContext, services) => {
            services
                .AddOptions()
                .Configure<mailsettings>(hostContext.Configuration.GetSection("mailsettings"))
                .AddHostedService<Worker>();
        });

Reference Configuration in ASP.NET Core

Reference Options pattern in ASP.NET Core