Validation and data annotations

ASP.NET Core validates models using data annotations and ModelState. Invalid submissions re-render forms with error messages—same idea as Django forms or Java Bean Validation.

Common attributes

• [Required], [StringLength(100)], [EmailAddress]
• [Range(1, 100)], [Compare("Password")]
• [RegularExpression] for custom patterns

public class LoginViewModel {
    [Required][EmailAddress]
    public string Email { get; set; } = "";
    [Required][DataType(DataType.Password)]
    public string Password { get; set; } = "";
}

Server-side check

if (!ModelState.IsValid)
    return View(model);

Client-side validation via jQuery Unobtrusive or similar is optional UX—never trust it alone.

Important interview questions and answers

1. Q: FluentValidation vs annotations?
   A: Annotations are simple on properties; FluentValidation suits complex rules in separate classes.
2. Q: API validation response?
   A: [ApiController] auto-returns 400 with ProblemDetails when ModelState invalid.

Self-check

1. What checks ModelState in an MVC action?
2. Why validate on the server even with client JS?

Tip: Re-display bound forms on validation failure so users do not retype everything—return View(model) with the same instance and show asp-validation-for messages.