How to do model field validation in asp.net c# MVC

Question

Please find the below model code. I want to do field validation. why the viewbag.msg doesn't show the error that [Required(ErrorMessage = "User is required for mobile model details")]

 

using System;

using System.Collections.Generic;

using System.ComponentModel.DataAnnotations;

using System.Linq;

using System.Web;

namespace SudentRegistration_MVC_Project.Models

{

public class Mobile

{

public int id { get; set; }

public string Mobile_brand { get; set; }

[Required(ErrorMessage = "User is required for mobile model details")]

public string mobile_model { get; set;}

public int mobile_price { get; set; }

public string mobile_description { get; set; }

public List MobileList { get; set; }

}

}