DI -在构造函数中自动初始化IOptions<T>

public class AppSettings
{
    public string ConnectionString { get; set; }
    public int MaxRetries { get; set; }
}

{
  "ConnectionStrings": {
    "DefaultConnection": "YourConnectionStringHere"
  },
  "AppSettings": {
    "ConnectionString": "YourConnectionStringHere",
    "MaxRetries": 3
  }
}

public class MyService
{
    private readonly AppSettings _appSettings;

    public MyService(IOptions<AppSettings> options)
    {
        _appSettings = options.Value;
    }

    public void DoSomething()
    {
        Console.WriteLine(_appSettings.ConnectionString);
        Console.WriteLine(_appSettings.MaxRetries);
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.Configure<AppSettings>(Configuration.GetSection("AppSettings"));
        services.AddTransient<MyService>();
    }
}

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

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.ConfigureAppConfiguration((context, config) =>
                {
                    config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);
                });
            })
            .ConfigureServices((hostContext, services) =>
            {
                services.Configure<AppSettings>(hostContext.Configuration.GetSection("AppSettings"));
                services.AddTransient<MyService>();
            });
}

public class MyService
{
    private readonly AppSettings _appSettings;

    public MyService(IOptions<AppSettings> options)
    {
        _appSettings = options.Value;
    }

    public void DoSomething()
    {
        Console.WriteLine(_appSettings.ConnectionString);
        Console.WriteLine(_appSettings.MaxRet.js);
    }
}