public class CustomEmailAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null || string.IsNullOrEmpty(value.ToString()))
        {
            return new ValidationResult("邮箱不能为空。");
        }

        var email = value.ToString();
        if (!email.Contains("@") || !email.Contains("."))
        {
            return new ValidationResult("邮箱格式不正确。");
        }

        return ValidationResult.Success;
    }
}

public class UserRegistrationModel
{
    [Required(ErrorMessage = "用户名是必填项")]
    public string UserName { get; set; }

    [CustomEmail]
    public string Email { get; set; }

    [Range(18, 100, ErrorMessage = "年龄必须在18到100之间")]
    public int Age { get; set; }
}

public class UserRegistrationModel : IValidatableObject
{
    public string Email { get; set; }
    public string ConfirmEmail { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        if (Email != ConfirmEmail)
        {
            yield return new ValidationResult("两次输入的邮箱不一致。", new[] { nameof(ConfirmEmail) });
        }
    }
}