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Mandatory model field in ASP.NET MVC

How to make a model field mandatory?

To mark a model property mandatory, we use [Required] attribute.

/Models/PersonalDetail.cs

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Web; 
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;  

namespace WebApplication1.Models
 {  
   public class PersonalDetail   
  {
         [Key]       
     [DatabaseGenerated(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOpt ion.Identity)]     
         public int AutoId { get; set; }  

        [StringLength(20, MinimumLength = 4, ErrorMessage = "Must be at least 4 characters long.")]            
        public string FirstName { get; set; }  

        [Required(ErrorMessage = "Please write your LastName")]          
        public string LastName { get; set; }  
     
        [Required]
        public int Age { get; set; }  

        [Display(Name = "Is Active?")]         
        public bool Active { get; set; }   
  }
 }  

Notice the highlighted attribute at the top. 

1. For LastName, we have written [Required] attribute with custom error message we want to show if user doens't input LastName field data.
2. For Age, the default error message comes when user doesn't input the data.