using System.ComponentModel.DataAnnotations;
public class User
{
    [Required(ErrorMessage = "用戶名不能為空")]
    [StringLength(20, MinimumLength = 3, ErrorMessage = "用戶名長度必須在3-20個字符之間")]
    public string Username { get; set; }
    [Required]
    [EmailAddress(ErrorMessage = "請輸入有效的電子郵件地址")]
    public string Email { get; set; }
    [Range(18, 100, ErrorMessage = "年齡必須在18-100歲之間")]
    public int Age { get; set; }
}

public class Order : IValidatableObject
{
    public DateTime OrderDate { get; set; }
    public DateTime? DeliveryDate { get; set; }
    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        if (DeliveryDate.HasValue && DeliveryDate < OrderDate)
        {
            yield return new ValidationResult(
                "交貨日期不能早于訂單日期",
                new[] { nameof(DeliveryDate) });
        }
    }
}

public static void ValidateModel(object model)
{
    var validationContext = new ValidationContext(model);
    var validationResults = new List<ValidationResult>();
    bool isValid = Validator.TryValidateObject(
        model, 
        validationContext, 
        validationResults,
        validateAllProperties: true);
    if (!isValid)
    {
        foreach (var result in validationResults)
        {
            Console.WriteLine($"{string.Join(",", result.MemberNames)}: {result.ErrorMessage}");
        }
        throw new ValidationException("數(shù)據(jù)驗(yàn)證失敗");
    }
}
// 使用示例
var user = new User { Username = "ab", Email = "invalid", Age = 17 };
ValidateModel(user);

public class ValidPhoneNumberAttribute : ValidationAttribute
{
    private static readonly Regex PhoneRegex = new Regex(@"^1[3-9]\d{9}$");
    protected override ValidationResult IsValid(object value, ValidationContext context)
    {
        if (value == null || !PhoneRegex.IsMatch(value.ToString()))
        {
            return new ValidationResult(ErrorMessage ?? "請輸入有效的手機(jī)號碼");
        }
        return ValidationResult.Success;
    }
}
// 使用自定義特性
public class Customer
{
    [ValidPhoneNumber]
    public string Phone { get; set; }
}