FluentValidationConditionalcheck.cs

RuleFor(x => x.FastDelivery)
                .NotEmpty()
                .WithState(x => new ValidationState() {
                    Code = "FastDelivery.empty",
                    Keys = { ValidationKeys.Delivery }
});

RuleFor(x => x.DeliveryDate)
               .NotEmpty()
               .WithState(x => new ValidationState()
               {
                   Code = "DeliveryDate.empty",
                   Keys = { ValidationKeys.Delivery }
               })
               .When(x=> x.FastDelivery == true);