How to extend the Subscribe form widget with server-side captcha validation?

In some specific use case scenarios there might be a need to have a captcha validation when the users subscribe to an email campaign through the Subscribe form widget.

In order to add a server-side captcha validation to the Subscribe form widget perform the steps below:

Create the custom Subscribe form widget template containing the RadCaptcha control:

1. Open the project in Visual Studio
2. Create a user control (.ascx file) titled SubscribeFormCustomTemplate.ascx
3. Paste the markup of the original markup of the Subscribe form widget template
    
   <%@ Control Language="C#" %>

   <%@ Register TagPrefix="sitefinity" Namespace="Telerik.Sitefinity.Web.UI" Assembly="Telerik.Sitefinity" %>

   <%@ Register TagPrefix="telerik" Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" %>

    

   <asp:Panel ID="errorsPanel" runat="server" CssClass="sfErrorSummary" Visible="false" />

   <fieldset id="formFieldset" runat="server" class="sfnewsletterForm sfSubscribe">

       <sitefinity:SitefinityLabel ID="widgetTitle" runat="server" WrapperTagName="h2" HideIfNoText="true" CssClass="sfnewsletterTitle" />

       <sitefinity:SitefinityLabel ID="widgetDescription" runat="server" WrapperTagName="p" HideIfNoText="true" CssClass="sfnewsletterDescription" />

       <sitefinity:Message ID="messageControl" runat="server" FadeDuration="3000" />

       <ol class="sfnewsletterFieldsList">

           <li class="sfnewsletterField">

               <asp:Label ID="emailAddressLabel" runat="server" Text='<%$Resources:NewslettersResources, EmailAddress %>' AssociatedControlID="emailAddress" CssClass="sfTxtLbl" />

               <asp:TextBox ID="emailAddress" runat="server" CssClass="sfTxt" />

               <asp:RequiredFieldValidator ID="emailValidator" runat="server" ControlToValidate="emailAddress" CssClass="sfErrorWrp" Display="Dynamic">

                   <strong class="sfError">

                       <asp:Literal runat="server" ID="lEmailIsRequired" Text='<%$Resources:NewslettersResources, EmailIsRequired %>' /></strong>

               </asp:RequiredFieldValidator>

               <asp:RegularExpressionValidator

                   ID="emailRegExp"

                   runat="server"

                   ControlToValidate="emailAddress"

                   ValidationExpression="[a-zA-Z0-9._%+-]+@(?:[a-zA-Z0-9-]+\.)+[a-zA-Z]{2,4}"

                   Display="Dynamic"

                   CssClass="sfErrorWrp"

                   ErrorMessage="<%$ Resources:ErrorMessages, EmailAddressViolationMessage %>">

                   <strong class="sfError">

                       <asp:Literal ID="lEmailNotValid" runat="server" Text="<%$ Resources:ErrorMessages, EmailAddressViolationMessage %>" /></strong>

               </asp:RegularExpressionValidator>

           </li>

           <li class="sfnewsletterField">

               <asp:Label ID="firstNameLabel" runat="server" Text='<%$Resources:NewslettersResources, FirstNamePublicForm %>' AssociatedControlID="firstName" CssClass="sfTxtLbl" />

               <asp:TextBox ID="firstName" runat="server" CssClass="sfTxt" />

           </li>

           <li class="sfnewsletterField">

               <asp:Label ID="lastNameLabel" runat="server" Text='<%$Resources:NewslettersResources, LastNamePublicForm %>' AssociatedControlID="lastName" CssClass="sfTxtLbl" />

               <asp:TextBox ID="lastName" runat="server" CssClass="sfTxt" />

           </li>       

       </ol>

       <div class="sfnewsletterSubmitBtnWrp">

           <asp:Button ID="subscribeButton" runat="server" Text='<%$Resources:NewslettersResources, SubscribeToList %>' CssClass="sfnewsletterSubmitBtn" />

       </div>

   </fieldset>

    

   <asp:Panel ID="selectListInstructionPanel" runat="server">

       <asp:Literal ID="pleaseSelectList" runat="server" Text='<%$Resources:NewslettersResources, ClickEditAndSelectList %>' />

   </asp:Panel>
   
    
4. Add the RadCaptcha control inside the <ol> element using the below markup:
    
   <li>

         <asp:Label ID="MyCaptchaLabel" runat="server" Text='Enter the text from the image' />

         <telerik:RadCaptcha  runat="server" ID="radCaptcha" EnableRefreshImage="true" CssClass="sfFormCaptcha" CaptchaTextBoxCssClass="sfTxt" CaptchaTextBoxLabelCssClass="sfTxtLbl"></telerik:RadCaptcha>

   </li>
   
    
5. Save the file

Create custom Subscribe form widget that uses the custom widget template created above and add server-side validation for the captcha control:

1. In Visual Studio create a class that inherits from the SubscribeForm class which is the built-in Subscribe form widget class.
2. Override the LayoutTemplatePath property and set it to the relative path of the custom template
3. Override the AddSubscriber() method where you can access the RadCaptcha control and validate it.
   - if it is not valid set the MessageControl to display the error message for the invalid captcha and return.
   - if it is valid call the base.AddSubscriber() method in order to add the subscriber
   
   Here is a sample code of the custom Subscribe form widget:
    
   using System;

   using System.Collections.Generic;

   using System.ComponentModel;

   using System.Linq;

   using System.Web;

   using Telerik.Sitefinity.Modules.Newsletters.Web.UI.Public;

   using Telerik.Sitefinity.Web.UI;

   using Telerik.Web.UI;

    

   namespace SitefinityWebApp

   {

       public class SubscribeFormCustom : SubscribeForm

       {

           /// <summary>

           /// Gets the reference to the RadCaptcha control from the template.

           /// </summary>

           protected virtual RadCaptcha RadCaptchaControl

           {

               get

               {

                   return this.Container.GetControl<RadCaptcha>("radCaptcha", true);

               }

           }

    

           // Specifies the template used by the