In this article I will explain with an example, how to perform Client Side validations using Data Annotation attribute in ASP.Net Core Razor Pages.
The Client Side validations will be performed using Model class and Data Annotation attributes.
 
 
Model
The following Model class consists of one property Gender.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 DropDownList_Validation_Razor_Core.Models
{
    public class PersonModel
    {
        [Required(ErrorMessage = "Gender is required.")]
        public string Gender { get; set; }
    }
}
 
 
Razor PageModel (Code-Behind)
The PageModel consists of following two Handler methods.
Handler method for handling GET operation
This Handler method handles the GET calls, for this particular example it is not required and hence left empty.
 
Handler method for handling POST operation
This Handler 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 Gender property is set in a ViewData object.
public class IndexModel : PageModel
{
    public PersonModel Person { get; set; }
    public void OnGet()
    {
    }
 
    public void OnPostSubmit(PersonModel person)
    {
        if (ModelState.IsValid)
        {
            ViewData["Gender"] = person.Gender;
        }
    }
}
 
 
Razor Page (HTML)
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.
The Form consists of a HTML DropDownList with three options, a SPAN and a Submit Button.
The DropDownList has been assigned with asp-for Tag Helper attribute and its value is set with the Gender property of the PersonModel object. Thus, this signifies that the validation will be performed for the Gender property.
The SPAN has been assigned with asp-validation-for Tag Helper attribute and here also its value is set with the Gender property of the PersonModel object which signifies that it is used for displaying the Validation message for the Gender property.
The Submit Button has been set with the POST Handler method using the asp-page-handler attribute.
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.
 
When the Submit button is clicked, the Form gets submitted and the Gender value is sent to the POST Handler method.
 
Enabling Client-Side validations
By default, the validations performed using Data Annotations and Model class is performed on Server Side.
In order to enable Client-Side validations, you will 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 of the ViewData object is displayed using Razor syntax.
@page
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@model DropDownList_Validation_Razor_Core.Pages.IndexModel
@{
    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">
        <table>
            <tr>
                <td>
                    <select asp-for="Person.Gender">
                        <option value="">Please select</option>
                        <option value="M">Male</option>
                        <option value="F">Female</option>
                    </select>
                </td>
                <td><span asp-validation-for="Person.Gender" class="error"></span></td>
            </tr>
            <tr>
                <td><input type="submit" value="Submit" asp-page-handler="Submit" /></td>
                <td></td>
            </tr>
        </table>
        <hr/>
        @if (ViewData.ModelState.IsValid)
        {
            @ViewData["Gender"];
        }
        <!--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>
    </form>
</body>
</html>
 
 
Screenshot
ASP.Net Core Razor Pages: Client Side DropDownList validation using jQuery
 
 
Downloads
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