using System;

using System.ComponentModel.DataAnnotations;

​

public class User

{

    public int Id { get; set; }

​

    [Required(ErrorMessage = "Username is required.")]

    [StringLength(50, MinimumLength = 3, ErrorMessage = "Username must be between 3 and 50 characters.")]

    public string Username { get; set; }

​

    [EmailAddress(ErrorMessage = "Invalid email address.")]

    public string Email { get; set; }

​

    [Range(18, 99, ErrorMessage = "Age must be between 18 and 99.")]

    public int Age { get; set; }

​

    [RegularExpression(@"^[A-Za-z\s]+$", ErrorMessage = "Only letters and spaces are allowed.")]

    public string FullName { get; set; }

}