public IEnumerable <ValidationResult> Validate(ValidationContext validationContext)
{
if (Birthdate > DateTimeOffset.Now)
{
yield return(new ValidationResult("A person cannot be born in the future.", new string[] { nameof(Birthdate) }));
}
if (EmailAddresses.GroupBy(e => e.Email).Select(grp => grp.Count()).Any(c => c > 1))
{
yield return(new ValidationResult("Email addresses must be unique.", new string[] { nameof(EmailAddresses) }));
}
}