Async validation?

[RichardD2]

I have a viewmodel which needs to call an external service to validate a property. As a contrived example:

[ObservableProperty]
[Required(ErrorMessage = "Please enter the reservation date/time")]
private DateTimeOffset? _reservationTime;

private async Task<ValidationResult> ValidateReservation()
{
    if (_reservationTime is not {} reservationTime) return ValidationResult.Success;
    
    var result = await MyService.ValidateReservation(reservationTime);
    if (result.CanMakeReservation) return ValidationResult.Success;
    
    return new(result.ErrorMessage, [nameof(ReservationTime)]);
}

However, I can't see any simple way to implement that using the ObservableValidator class. There doesn't seem to be any support for async validation in the System.ComponentModel.DataAnnotations, and I can't see any way for a validation task to add a validation error message for a property.

The best I can come up with is a horrible hack:

[ObservableProperty]
[Required(ErrorMessage = "Please enter the reservation date/time")]
[CustomValidation(typeof(MyViewModel), nameof(ValidateReservationTime))]
private DateTimeOffset? _reservationTime;

private ValidationResult? _reservationTimeError;

public static ValidationResult ValidateReservationTime(DateTimeOffset? value, ValidationContext context)
{
    return ((MyViewModel)context.ObjectInstance)._reservationTimeError;
}

partial void OnReservationTimeChanged(DateTimeOffset? value)
{
    _ = UpdateReservationTimeValidation();
}

private async Task UpdateReservationTimeValidation()
{
    _reservationTimeError = await ValidateReservation();
    ValidateProperty(ReservationTime, nameof(ReservationTime));
}

Have I missed something obvious?