RegisterPageViewModel.cs

public class RegisterPageViewModel: INotifyPropertyChanged
{
        public ValidatableObject<string> FirstName { get; set; } = new ValidatableObject<string>();
        public ValidatableObject<string> LastName { get; set; } = new ValidatableObject<string>();
        public ValidatableObject<DateTime> BirthDay { get; set; } = new ValidatableObject<DateTime>() { Value = DateTime.Now };
        public ValidatableObject<string> PhoneNumber { get; set; } = new ValidatableObject<string>();
        public ValidatablePair<string> Email { get; set; } = new ValidatablePair<string>();
        public ValidatablePair<string> Password { get; set; } = new ValidatablePair<string>();
        public ValidatableObject<bool> TermsAndCondition { get; set; } = new ValidatableObject<bool>();
  }