Custom Model Validation In ASP.NET Core 3.1

using MvcCoreCustomModelValidation_Demo.CustomValidation;  
using System;  
using System.ComponentModel.DataAnnotations;  
namespace MvcCoreCustomModelValidation_Demo.Models  
{  
    public class Student  
    {  
        [Key]  
        public int Id { get; set; }  
        [Required(ErrorMessage = "Please enter name")]  
        public string Name { get; set; }  
        [Required(ErrorMessage = "Please choose admission date.")]  
        [Display(Name = "Admission Date")]  
        [DataType(DataType.Date)]  
        [CustomAdmissionDate(ErrorMessage = "Admission Date must be less than or equal to Today's Date.")]  
        public DateTime AdmissionDate { get; set; }  
        [Display(Name = "Date of Birth")]  
        [DataType(DataType.Date)]  
        [Min18Years]  
        public DateTime DateofBirth { get; set; }  
    }  
}  

Step 3. Right-click on the project and “Add” folder CustomValidation. Now “Add” two classes, CustomAdmissionDate and Min18Years, respectively. Inherit from ValidationAttribute class override method IsValid with bool property.

using System;  
using System.ComponentModel.DataAnnotations;  
namespace MvcCoreCustomModelValidation_Demo.CustomValidation  
{  
    public class CustomAdmissionDate : ValidationAttribute  
    {  
        public override bool IsValid(object value)  
        {  
            DateTime dateTime = Convert.ToDateTime(value);  
            return dateTime <= DateTime.Now;  
        }  
    }  
}  

using MvcCoreCustomModelValidation_Demo.Models;  
using System;  
using System.ComponentModel.DataAnnotations;  
namespace MvcCoreCustomModelValidation_Demo.CustomValidation  
{  
    public class Min18Years : ValidationAttribute  
    {  
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)  
        {  
            var student = (Student)validationContext.ObjectInstance;  
            if (student.DateofBirth == null)  
                return new ValidationResult("Date of Birth is required.");  
            var age = DateTime.Today.Year - student.DateofBirth.Year;  
            return (age >= 18)  
                ? ValidationResult.Success  
                : new ValidationResult("Student should be at least 18 years old.");  
        }  
    }  
}  

Step 4. Now open HomeController which was added when we created the new project. Write the following code for New IActionResult methods.

using Microsoft.AspNetCore.Mvc;  
using MvcCoreCustomModelValidation_Demo.Models;  
namespace MvcCoreCustomModelValidation_Demo.Controllers  
{  
    public class HomeController : Controller  
    {  
        public IActionResult Index()  
        {  
            return View();  
        }  
        public IActionResult New()  
        {  
            return View();  
        }  
        [HttpPost]  
        [ValidateAntiForgeryToken]  
        public IActionResult New(Student student)  
        {  
            if (ModelState.IsValid)  
            {  
                RedirectToAction("Index");  
            }  
            return View();  
        }  
    }  
}  

Step 5. Right-click on the New IActionResult method. “Add” view with default name “New”. Write the following code.

@model MvcCoreCustomModelValidation_Demo.Models.Student  
  
@{  
    ViewData["Title"] = "New";  
}  
  
<div class="card">  
    <div class="card-header">  
        <h4 class="text-uppercase">Student Information</h4>  
    </div>  
    <div class="card-body">  
        <form asp-action="New">  
            <div class="form-group">  
                <label asp-for="Name" class="label-control"></label>  
                <input asp-for="Name" class="form-control" />  
                <span asp-validation-for="Name" class="text-danger"></span>  
            </div>  
            <div class="row">  
                <div class="col-md-6">  
                    <div class="form-group">  
                        <label asp-for="AdmissionDate" class="label-control"></label>  
                        <input asp-for="AdmissionDate" class="form-control" />  
                        <span asp-validation-for="AdmissionDate" class="text-danger"></span>  
                    </div>  
                </div>  
                <div class="col-md-6">  
                    <div class="form-group">  
                        <label asp-for="DateofBirth" class="label-control"></label>  
                        <input asp-for="DateofBirth" class="form-control" />  
                        <span asp-validation-for="DateofBirth" class="text-danger"></span>  
                    </div>  
                </div>  
            </div>  
            <div class="form-group">  
                <button type="submit" class="btn btn-sm btn-primary rounded-0">Submit</button>  
            </div>  
        </form>  
    </div>  
</div>  

Step 6. Built and run the application; ctrl+F5.