public IEnumerable <ValidationResult> Validate(ValidationContext validationContext)
{
if (!General.Validator.IsOnlyLetters(Author.Replace(" ", "")))
{
yield return
(new ValidationResult(errorMessage: "Author contains only letters",
memberNames: new[] { "Author" }));
}
if (ReleaseDate.Date > DateTime.Now.Date)
{
yield return
(new ValidationResult(errorMessage: "Date can not be greater than today's date",
memberNames: new[] { "ReleaseDate" }));
}
}