bool TryGetDomain(string? url, out string? domain)
{
    if (url is null || !url.Contains("://"))
    {
        domain = null;
        return false;
    }

    domain = url.Split("://")[1];
    return true;   // здесь domain гарантированно не null
}

if (TryGetDomain(link, out var d))
{
    Console.WriteLine(d.Length);   // Предупреждение CS8602 «возможен null»
}

bool TryGetDomain(
        string? url,
        [NotNullWhen(true)] out string? domain)
{
    if (url is null || !url.Contains("://"))
    {
        domain = null;
        return false;  // при false null допустим
    }

    domain = url.Split("://")[1];
    return true;   // при true null невозможен
}

if (TryGetDomain(link, out var d))
{
    Console.WriteLine(d.Length);   // без варнингов и лишних проверок
}

[HttpPost("create")]
public IActionResult Create(OrderDto dto)
{
    var customer = _db.Customers.Find(dto.CustomerId);
    var now = DateTime.Now;
    decimal subtotal = dto.Lines.Sum(l => l.Price * l.Qty);

    // Скидка на черную пятницу
    if (now.Month == 11 && now.Day >= 25 && now.Day <= 30)
        subtotal *= 0.8m;

    decimal vat = subtotal * 0.12m;
    decimal total = subtotal + vat;

    customer.Balance -= total;
    _db.SaveChanges();

    _smtp.Send("sales@corp", customer.Email,
                "Thanks for your order", $"Total = {total:C}");

    _log.Write($"{now:u} Order {dto.Id} for {customer.Id} = {total}");

    return Ok(new { id = dto.Id, total });
}

public class OrderPricer
{
    private readonly IClock _clock;
    private const decimal VatRate = 0.12m;

    //  в .NET 8 и выше можно использовать встроенный TimeProvider вместо IClock
    public OrderPricer(IClock clock) => _clock = clock;

    public decimal CalculateTotal(IEnumerable<OrderLineDto> lines)
    {
        decimal subtotal = lines.Sum(l => l.Price * l.Qty);
        var now = _clock.UtcNow;

        // Скидка на черную пятницу
        if (now.Month == 11 && now.Day >= 25 && now.Day <= 30)
            subtotal *= 0.8m;

        decimal vat = subtotal * VatRate;
        return subtotal + vat;
    }
}

public class FixedClock : IClock
{
    public DateTime UtcNow { get; init; }
}

public class OrderPricerTests
{
    [Fact]
    public void CalculatesTotalWithVat_NoDiscount()
    {
        var clock = new FixedClock { UtcNow = new DateTime(2025, 7, 19) };
        var pricer = new OrderPricer(clock);

        var total = pricer.CalculateTotal(new[]
        {
            new OrderLineDto { Price = 100, Qty = 1 }
        });

        Assert.Equal(112m, total);
    }
    // остальные тесты
}

[HttpPost("create")]
public IActionResult Create(OrderDto dto)
{
    var customer = _db.Customers.Find(dto.CustomerId);
    var pricer = new OrderPricer(_clock);
    decimal total = pricer.CalculateTotal(dto.Lines);

    customer.Balance -= total;
    _db.SaveChanges();

    _smtp.Send("sales@corp", customer.Email,
                "Thanks for your order", $"Total = {total:C}");
    _log.Write($"{_clock.UtcNow:u} Order {dto.Id} for {customer.Id} = {total}");

    return Ok(new { id = dto.Id, total });
}

{
  "FeatureManagement": {
    //"NewDashboard": true // Случайно закомментированный или удаленный параметр
  }
}

public class FeaturesConfig
{
    public bool NewDashboard { get; set; }
}

public class FeaturesConfig
{
    [Required]
    [NotNull]
    public bool? NewDashboard { get; set; }
}

builder.Services.AddOptions<FeaturesConfig>()
    .Bind(builder.Configuration.GetSection(nameof(FeaturesConfig)))
    .ValidateDataAnnotations()
    .ValidateOnStart();