In this article I will explain with an example, how to implement TextBox validation in ASP.Net Core MVC.
The TextBox validation will be performed on Client Side as well as Server Side using Data Annotations in ASP.Net Core MVC.
 
 
Model
The following Model class consists of one property Name. The property is decorated with the following Data Annotation attributes for performing validations.
1. Required Data Annotation attribute.
The Data Annotations attributes can be used with the Entity Data Model (EDM), LINQ to SQL, and other data models.
The Required Data Annotation has been specified with a property Error Message with a string value. As the name suggests, this string value will be displayed to the user when the validation fails.
using System.ComponentModel.DataAnnotations;
 
namespace TextBox_Validation_MVC_Core.Models
{
    public class PersonModel
    {
        [Required(ErrorMessage = "Name is required.")]
        public string Name { get; set; }
    }
}
 
 
Controller
The Controller consists of two Action methods.
Action method for handling GET operation
Inside this Action method, simply the View is returned.
 
Action method for handling POST operation
This action method handles the POST operation and when the form is submitted, the object of the PersonModel class is sent to this method.
The state of the submitted Model is checked using ModelState.IsValid property and if the Model is valid then the value of the Name property is set in a ViewBag object.
public class HomeController : Controller
{
    public IActionResult Index()
    {
        return View();
    }
 
    [HttpPost]
    public IActionResult Index(PersonModel person)
    {
        if (ModelState.IsValid)
        {
            ViewBag.Name = person.Name;
        }
 
        return View();
    }
}
 
 
View
Inside the View, in the very first line the PersonModel class is declared as Model for the View.
 
The Form
The View consists of an HTML Form which has been created using following ASP.Net Tag Helpers attributes.
asp-action – Name of the Action. In this case the name is Index.
asp-controller – Name of the Controller. In this case the name is Home.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
The Form consists of a HTML TextBox, a SPAN and a Submit Button.
The TextBox has been assigned with asp-for Tag Helper attribute and its value is set with the Name property. Thus, this signifies that the validation will be performed for the Name property.
The SPAN has been assigned with asp-validation-for Tag Helper attribute and here also its value is set with the Name property which signifies that it is used for displaying the Validation message for the Name property.
When the Submit button is clicked, the Form gets submitted and the Name value is sent to the Controller.
 
Enabling Client-Side validations
The default validation performed is server side when one uses Data Annotations along with Model class for validation purpose.
In order to enable Client-Side validations, you need to inherit the following script files.
1. jquery.js
2. jquery.validate.js
3. jquery.validate.unobtrusive.js
Once, the above files are inherited automatically the Client-Side validations using Data Annotations is enabled.
 
Displaying submitted value
Below the Form, the ModelState.IsValid property is checked and if the Model is valid, then the value if the ViewBag object is displayed using Razor syntax.
@model TextBox_Validation_MVC_Core.Models.PersonModel
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@{
    Layout = null;
}
 
<!DOCTYPE html>
 
<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
    <style type="text/css">
        body { font-family: Arial; font-size: 10pt; }
        .error { color: red; }
    </style>
</head>
<body>
    <form method="post" asp-controller="Home" asp-action="Index">
        <table>
            <tr>
                <td><input asp-for="Name" /></td>
                <td><span asp-validation-for="Name" class="error"></span></td>
            </tr>
            <tr>
                <td><input type="submit" value="Submit" /></td>
                <td></td>
            </tr>
        </table>
    </form>
    <hr/>
    @if (ViewData.ModelState.IsValid)
    {
        @ViewBag.Name;
    }
    <!--OPTIONAL: Add the following scripts for enabling Client Side validation.-->
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.19.3/jquery.validate.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validation-unobtrusive/3.2.12/jquery.validate.unobtrusive.js"></script>
</body>
</html>
 
 
Screenshot
TextBox validation in ASP.Net Core
 
 
Downloads