MVC validation exemple

UsersController.cs

using System.ComponentModel.DataAnnotations;
using System.IO;
using System.Linq;
using System.Net;
using System.Web.Mvc;
using Newtonsoft.Json;

namespace WebApplication1.Controllers
{
    // It a DTO model
    public class UserInfo
    {
        [Required]
        [StringLength(60, MinimumLength = 3)]
        public string Login { get; set; }

        [Required]
        [StringLength(maximumLength: 20, MinimumLength = 6, ErrorMessage = "Password length must be between 6 and 20.")]
        public string Password { get; set; }

        [Required]
        [RegularExpression(@"^.*@.*$", ErrorMessage = "Invalid email")]
        public string Email { get; set; }

        [Required(ErrorMessage = "Please specify a birth year")]
        public int BirthYear { get; set; }
    }

    // Custom ActionResult
    public class ApiError : ActionResult
    {
        private readonly ModelStateDictionary _model;

        public ApiError(ModelStateDictionary model)
        {
            _model = model;
        }

        public override void ExecuteResult(ControllerContext context)
        {
            var errors = _model.Values
                    .SelectMany(v => v.Errors.Select(e => e.ErrorMessage))
                    .ToList();

            var json = JsonConvert.SerializeObject(errors);
            var outputStream = context.HttpContext.Response.OutputStream;
            context.HttpContext.Response.StatusCode = (int) HttpStatusCode.BadRequest;
            context.HttpContext.Response.ContentType = "application/json";
            using (var w = new StreamWriter(outputStream))
            {
                w.Write(json);
                w.Flush();
                outputStream.Flush();
            }
        }
    }
    // Controller Extension
    public static class ControllerExtensions
    {
        public static ActionResult ApiError(this Controller controller, ModelStateDictionary modelState) 
            => new ApiError(modelState);
    }

    // Controller
    public class UsersController : Controller
    {
        [HttpPost]
        public ActionResult Save(UserInfo info)
        {
            if (!ModelState.IsValid)
                return this.ApiError(ModelState);

            return Json(true);
        }
    }
}