Deep-Dive: Data Validation & Data Annotations

Never trust user input. Professional validation includes both Client-Side and Server-Side checks. We prevent "Garbage Data" from ever hitting our SQL Server.

Section 1: Built-in Data Annotations

public class RegisterViewModel {
    [Required(ErrorMessage = "Full Name is mandatory.")]
    [StringLength(100, MinimumLength = 3)]
    public string FullName { get; set; }

    [EmailAddress]
    [Required]
    public string Email { get; set; }

    [Range(18, 99)]
    public int Age { get; set; }
}
                

Section 2: Custom Validation Attributes (Expert Level)

What if you need to validate that a Date is in the past? Or that a username is unique? This requires a custom attribute.

public class PastDateAttribute : ValidationAttribute {
    protected override ValidationResult IsValid(object value, ValidationContext context) {
        if (value is DateTime dt && dt >= DateTime.Now) {
            return new ValidationResult("Date must be in the past!");
        }
        return ValidationResult.Success;
    }
}