In this article I will explain with an example, how to perform Mobile Number (Cellphone Number) validation in TextBox i.e. exact Ten Numbers (Digits) validation on Client Side using Data Annotation in ASP.Net Core Razor Pages.
Note: For beginners in ASP.Net Core Razor Pages (.Net Core 7), please refer my article ASP.Net Core 7 Razor Pages: Hello World Tutorial with Sample Program example.
 
The Mobile Phone Number (Cellphone Number) validation will be performed using Regular Expression (Regex).
 
 

Model

The following Model class consists of one property MobileNumber. The property is decorated with the following Data Annotation attributes for performing validations.
1. Required Data Annotation attribute.
2. RegularExpression Data Annotation attribute.
Note: The Data Annotations attributes can be used with the Entity Data Model (EDM), LINQ to SQL, and other data models.
 
The Required Data Annotation and the RegularExpression Data Annotation attributes have 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 respective validation fails.
The RegularExpression Data Annotation attribute has been set with a Regular Expression for validating Mobile Phone Number (Cellphone Number).
using System.ComponentModel.DataAnnotations;
 
namespace MobileNumber_Validation_Razor_Core.Models
{
    public class PersonModel
    {
        [Display(Name = "Mobile Number:")]
        [Required(ErrorMessage = "Mobile Number is required.")]
        [RegularExpression("^([0-9]{10})$", ErrorMessage = "Invalid Mobile Number.")]
        public string MobileNumber { get; set; }
    }
}
 
 

Razor PageModel (Code-Behind)

The PageModel consists of following Handler methods.

Handler method for handling GET operation

This Handler method left empty as it is not required.
 

Handler method for handling POST operation

This Handler method handles the POST operation and when the form is submitted.
The state of the submitted Model is checked using ModelState.IsValid property.
Note: ModelState.IsValid property is an inbuilt property which verifies two things:
          1. Whether the Form values are bound to the Model.
          2. All the validations specified inside Model class using Data annotations have been passed.
 
public class IndexModel : PageModel
{
    public PersonModel Person { get; set; }
    public void OnGet()
    {
    }
 
    public void OnPostSubmit()
    {
        if (ModelState.IsValid)
        {
            // Validation Success.
        }
    }
}
 
 

Razor Page (HTML)

HTML Markup

The Form

The HTML of Razor Page consists of an HTML Form which has been created using following ASP.Net Tag Helpers attribute.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
 

Implementing Validation

The Form consists of a Label, an HTML INPUT TextBox, a SPAN element and a Submit Button.
The TextBox has been set with the following Tag Helpers attributes:-
asp-for – The Model property to which validation will be performed. In this case MobileNumber.
asp-validation-for – Displaying the validation message for the Model property.
The Submit Button has been set with the POST Handler method using the asp-page-handler attribute.
When the Submit Button is clicked, the Form gets submitted and the TextBox value i.e. MobileNumber is sent to the Handler method.
Note: In the Razor PageModel, the Handler method name is OnPostSubmit but here it will be specified as Submit when calling from the Razor HTML Page.
 

Enabling Client-Side validations

The default validation performed on server side when one uses Data Annotations along with Model class for validation purpose.
In order to enable Client-Side validations, you will need to inherit the following script files.
1. jquery.min.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.
@page
@model MobileNumber_Validation_Razor_Core.Pages.IndexModel
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@{
    Layout = null;
}
 
<!DOCTYPE html>
 
<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
</head>
<body>
    <form method="post">
        <table>
            <tr>
                <td><label asp-for="Person.MobileNumber"></label></td>
                <td><input type="text"asp-for="Person.MobileNumber"/></td>
                <td><span asp-validation-for="Person.MobileNumber" class="error"></span></td>
            </tr>
            <tr>
                <td></td>
                <td><input type="submit" value="Submit" asp-page-handler="Submit" /></td>
                <td></td>
            </tr>
        </table>
    </form>
 
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.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

ASP.Net Core Razor Pages: Mobile Number (Cellphone Number) Validation using Data Annotations
 
 

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