DataAnnotations & IValidatableObject

The schema generator already reads the attributes it can turn into field rules, so [Required] and [Range] need nothing extra. What no attribute can express is a rule that reads more than one property — check out after check in, guests within the room's capacity — and in .NET that is IValidatableObject. UseDataAnnotations() runs it, exactly as UseFluentValidation() runs a FluentValidation validator — and the two compose, so a model that carries both gets both. The rule about single spaces in the guest's name is a FluentValidation rule sitting alongside the model's own; a complaint the two layers would word identically on the same field is still read once.

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public class RoomBooking : IValidatableObject
{
    [Required, Display(Name = "Guest name")] public string GuestName { get; set; } = "";
    [Required, EmailAddress]                 public string Email     { get; set; } = "";
    public DateOnly CheckIn  { get; set; }
    public DateOnly CheckOut { get; set; }
    [Range(1, 8)] public int Guests { get; set; } = 1;
    public string RoomType { get; set; } = "single";

    // The rules that need more than one property — which is precisely what an attribute cannot see.
    public IEnumerable<ValidationResult> Validate(ValidationContext ctx)
    {
        if (CheckOut <= CheckIn)
            yield return new("Check out must be after check in.", [nameof(CheckOut)]);

        var capacity = RoomType switch { "single" => 1, "double" => 2, _ => 4 };
        if (Guests > capacity)
            // Naming two members puts the same complaint under both, so whichever one the user is
            // looking at explains itself.
            yield return new($"A {RoomType} room sleeps {capacity}.", [nameof(Guests), nameof(RoomType)]);

        // Naming no member at all is a statement about the booking as a whole, and lands on the form.
        if ((CheckOut.DayNumber - CheckIn.DayNumber) > 28)
            yield return new("Stays longer than 28 nights have to be booked by phone.");
    }
}

// The attributes are already field rules through the generator, so only the cross-property layer is
// added — running Validator over the properties again would put two differently-worded copies of
// "required" under the same box. Pass includePropertyAttributes: true when the schema came from
// somewhere that never saw them (a JSON Schema document rendered over a typed model).
var state = new BormState(BormSchemaGenerator.Generate<RoomBooking>(),
                          new BormModelDataAccessor(model))
    // The two integrations combine rather than replace each other, so a model whose rules are split
    // across both layers gets both. Either order works.
    .UseDataAnnotations()
    .UseFluentValidation(new RoomBookingValidator());

public sealed class RoomBookingValidator : AbstractValidator<RoomBooking>
{
    public RoomBookingValidator()
        => RuleFor(x => x.GuestName)
            .Must(name => !name.Contains("  "))
            .WithMessage("Please use single spaces in the guest's name.");
}

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