In this article I will explain with an example, how to perform Server Side Email validation using Data Annotation attribute in ASP.Net Core Razor Pages.
The Server Side Email validation will be performed using Model class and Data Annotation attributes.
 
 
Model
The following Model class consists of one property Email. The property is decorated with the following Data Annotation attributes for performing validations.
1. Required Data Annotation attribute.
2. EmailAddress 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 and the EmailAddress 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.
using System.ComponentModel.DataAnnotations;
 
namespace Email_Validation_Server_Side_Razor_Core.Models
{
    public class PersonModel
    {
        [Required(ErrorMessage = "Email is required.")]
        [EmailAddress(ErrorMessage = "Invalid Email Address.")]
        public string Email { 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.
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 string Email { get; set; }
    public void OnGet()
    {
    }
 
    public void OnPostSubmit(PersonModel person)
    {
        if (ModelState.IsValid)
        {
            // Validation success.
        }
    }
}
 
 
Razor Page (HTML)
Inside the HTML of Razor Page, in the very first line the PersonModel class is declared as Model for the View.
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 INPUT TextBox, a SPAN element and a Submit Button.
The INPUT TextBox has been assigned with asp-for Tag Helper attribute and its value is set with the Email property. Thus, this signifies that the validation will be performed for the Email property.
The SPAN has been assigned with asp-validation-for Tag Helper attribute and here also its value is set with the Email property which signifies that it is used for displaying the validation message for the Email 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.
 
@page
@model Email_Validation_Server_Side_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>
    <style type="text/css">
        body { font-family: Arial; font-size: 10pt; }
        .error { color: red; }
    </style>
</head>
<body>
    <form method="post">
        <table>
            <tr>
                <td><input asp-for="Email" /></td>
                <td><span asp-validation-for="Email" class="error"></span></td>
            </tr>
            <tr>
                <td><input type="submit" value="Submit" asp-page-handler="Submit" /></td>
                <td></td>
            </tr>
        </table>
    </form>
</body>
</html>
 
 
Screenshot
ASP.Net Core Razor Pages: Server Side Email Validation using Data Annotation attributes
 
 
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