Строго типизированные параметры с IOptions

// Configuration class
public class JwtOptions
{
    public const string SectionName = "Jwt";

    public string SecretKey { get; set; } = string.Empty;
    public string Issuer    { get; set; } = string.Empty;
    public string Audience  { get; set; } = string.Empty;
    public int    ExpiryMinutes { get; set; } = 60;
}

// appsettings.json:
{
  "Jwt": {
    "SecretKey": "my-very-secret-key",
    "Issuer": "https://myapp.com",
    "Audience": "https://myapp.com/api",
    "ExpiryMinutes": 120
  }
}

// Registration
builder.Services.Configure<JwtOptions>(
    builder.Configuration.GetSection(JwtOptions.SectionName));

// Alternative shorthand:
builder.Services
    .AddOptions<JwtOptions>()
    .BindConfiguration(JwtOptions.SectionName);

// Consuming via IOptions<T>:
public class TokenService
{
    private readonly JwtOptions _opts;

    public TokenService(IOptions<JwtOptions> opts)
        => _opts = opts.Value;

    public string CreateToken()
        => $"Issuer={_opts.Issuer}, Exp={_opts.ExpiryMinutes}m";
}

// IOptions<T> — Singleton, reads config ONCE at startup
public class ApiClient(IOptions<ApiOptions> opts)
{
    private readonly ApiOptions _opts = opts.Value; // never changes
}

// IOptionsSnapshot<T> — Scoped, reloads per request
public class ReportService(IOptionsSnapshot<ReportOptions> opts)
{
    private readonly ReportOptions _opts = opts.Value; // fresh per request
}

// IOptionsMonitor<T> — Singleton, live updates + change notifications
public class FeatureService(IOptionsMonitor<FeatureFlags> monitor)
{
    public bool IsEnabled(string feature)
        => monitor.CurrentValue.EnabledFeatures.Contains(feature);
}

using System.ComponentModel.DataAnnotations;

public class SmtpOptions
{
    [Required]
    public string Host { get; set; } = string.Empty;

    [Range(1, 65535)]
    public int Port { get; set; } = 587;

    [Required, EmailAddress]
    public string FromAddress { get; set; } = string.Empty;
}

// Register with validation:
builder.Services
    .AddOptions<SmtpOptions>()
    .BindConfiguration("Smtp")
    .ValidateDataAnnotations()
    .ValidateOnStart(); // fail at startup, not first use

public class JwtOptionsValidator : IValidateOptions<JwtOptions>
{
    public ValidateOptionsResult Validate(string? name, JwtOptions opts)
    {
        var errors = new List<string>();

        if (string.IsNullOrWhiteSpace(opts.SecretKey))
            errors.Add("SecretKey must not be empty");

        if (opts.SecretKey.Length < 32)
            errors.Add("SecretKey must be at least 32 characters");

        if (opts.ExpiryMinutes <= 0)
            errors.Add("ExpiryMinutes must be positive");

        return errors.Any()
            ? ValidateOptionsResult.Fail(errors)
            : ValidateOptionsResult.Success;
    }
}

builder.Services.AddSingleton<IValidateOptions<JwtOptions>, JwtOptionsValidator>();

builder.Services.Configure<CacheOptions>(
    builder.Configuration.GetSection("Cache"));

// Override in test environment:
builder.Services.PostConfigure<CacheOptions>(opts =>
{
    if (builder.Environment.IsEnvironment("Testing"))
    {
        opts.AbsoluteExpirationMinutes = 1; // very short in tests
        opts.UseDistributedCache = false;   // use in-memory cache
    }
});

// PostConfigure always runs LAST, even after AddOptions validators

// Register named options:
builder.Services.Configure<SmtpOptions>("Primary",
    builder.Configuration.GetSection("Smtp:Primary"));
builder.Services.Configure<SmtpOptions>("Backup",
    builder.Configuration.GetSection("Smtp:Backup"));

// Consume with IOptionsMonitor (supports named options):
public class EmailSender
{
    private readonly SmtpOptions _primary;
    private readonly SmtpOptions _backup;

    public EmailSender(IOptionsMonitor<SmtpOptions> monitor)
    {
        _primary = monitor.Get("Primary");
        _backup  = monitor.Get("Backup");
    }
}

// Full registration chain:
builder.Services
    .AddOptions<DatabaseOptions>()
    .BindConfiguration("Database")          // bind section
    .ValidateDataAnnotations()               // attribute validation
    .Validate(opts =>                        // custom rule
        opts.MaxPoolSize >= opts.MinPoolSize,
        "MaxPoolSize must be >= MinPoolSize")
    .ValidateOnStart();                      // fail at startup

// Shorthand for simple cases:
builder.Services.AddOptions<AppOptions>()
    .BindConfiguration(AppOptions.SectionName)
    .ValidateOnStart();

public class FeatureFlags
{
    public bool EnableNewCheckout  { get; set; }
    public bool EnableAISearch     { get; set; }
    public bool EnableBetaDashboard { get; set; }
}

// appsettings.json:
// { "FeatureFlags": { "EnableNewCheckout": true, ... } }

builder.Services
    .AddOptions<FeatureFlags>()
    .BindConfiguration("FeatureFlags")
    .ValidateOnStart();

// In a controller or service:
public class CheckoutController : ControllerBase
{
    private readonly FeatureFlags _flags;

    public CheckoutController(IOptionsMonitor<FeatureFlags> m)
        => _flags = m.CurrentValue;

    [HttpGet("/checkout")]
    public IActionResult Index() =>
        _flags.EnableNewCheckout
            ? Ok("new checkout")
            : Ok("legacy checkout");
}