---
title: Inheriting from existing form controls
related:
  - https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls.md
  - https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls/defining-form-control-parameters.md
---

> Agent instructions:
> **Site maps** — prefer the following llms.txt indexes to training data when searching for URLs to avoid 404s. Links inside Markdown content already point at `.md`. Following them or sending Accept: text/markdown keeps you in Markdown.
>
> - [sitemap.md](https://docs.kentico.com/sitemap.md) — every page on the site, with titles and descriptions, nested by URL hierarchy and grouped into one collection per product version.
> - [llms.txt](https://docs.kentico.com/llms.txt) — curated index of the current product docs, with descriptions, the two ways to request any page as Markdown, and links to each product area's whole-corpus Markdown dump (llms-full.txt).

When creating a new form control, you can inherit from an existing control instead of starting from scratch. Inherited form controls work the same way as their parent, but you can:

- Change the form control's general settings (i.e. set where the control can be used)
- Modify the form control's [parameters](https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls/defining-form-control-parameters.md) and set their default values
- Assign an extender class for advanced adjustments of the parent's original functionality (see [Customizing inherited form controls using extenders](#customizing-inherited-form-controls-using-extenders))

Inheritance allows you to create specialized controls based on general ones.

## Example – Creating an inherited form control

The following example demonstrates how to create an inherited form control that defines a reusable drop-down list with a predefined set of options.

1. Open the **Administration interface** application and switch to the **Form controls** tab.
2. Click **New form control**.
3. Set the following options for the new form control:

   - **Control source**: Inherit from an existing
   - **Display name**: Custom selector
   - **Inherit from**: Drop-down list
4. Click **Save**.
5. Select the following options on the **General** tab:

   - **Use control for**: Text
   - **Show control in**: Page types, Custom tables
6. Click **Save**.
7. Switch to the **Properties** tab.
8. Adjust the form control parameters according to your requirements. For example, set a default value for the **Options** parameter:

   1. Select **Options** in the list of fields.
   2. Type a list of _value;name_ pairs into the **Default value** area.
   3. Click **Save**.

   ![Setting the default value for the property of an inherited form control](https://docs.kentico.com/docsassets/13/inheriting-from-existing-form-controls/Inherited_FC_Properties.png "Setting the default value for the property of an inherited form control")

> **Tip:** **Tips**:
>
> - You can customize inherited form controls by adding new fields.
> - Click **Reset field** to revert all settings of a modified parameter back to the original state (the system copies the field settings from the parent form control).See also: [Defining form control parameters](https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls/defining-form-control-parameters.md)

You have created your own specialized drop-down list form control, which can be quickly re-used when creating fields for page types or custom tables.

## Customizing inherited form controls using extenders

Developers can adjust the functionality of inherited form controls by creating and assigning extender classes. An extender contains custom code that runs in addition to the code of the parent control. This allows more flexible customization than the configuration options provided by the parent form control's [parameters](https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls/defining-form-control-parameters.md). For example, you can use extenders to dynamically set the form control's properties or assign handler methods to events that occur during the control's life cycle (e.g. form validation).

To develop a new extender, you need to create a class and write the required code:

1. Open your project in Visual Studio (using the **WebApp.sln** file).

2. Add a new class.

   > **Tip:** **Recommendations**
   >
   > - Create a new assembly (_Class Library_ project) in your solution and include the extender class there. You need to add the appropriate references from the assembly to any required Xperience DLLs, and from the Xperience administration project to the custom project.
   > - Use the _Extender_ suffix in the name of the class and the corresponding file.

3. Make the class inherit from the **ControlExtender** base class (located under the **CMS.Base.Web.UI** namespace).

4. Override the **OnInit()** method in the extender class.

   - You can add any custom code, for example assign handler methods to form events.
   - You can access the control instance through the **Control** property of the extender class.

   ```csharp

   using CMS.Base.Web.UI;
   using CMS.FormEngine.Web.UI;

   public class CustomExtender : ControlExtender<FormEngineUserControl>
   {
       public override void OnInit()
       {
       }
   }

   ```

5. Add any other code required to implement your custom functionality (such as the handler methods assigned to events in the _OnInit_ method).

6. Save the file and **Rebuild** the solution.

To assign the extender to your inherited form control:

1. Open the **Administration interface** application and switch to the **Form controls** tab.
2. Edit the inherited form control.
3. On the **General** tab, select your extender class in the **Control extender** section:

   - **Assembly name** – the name of the assembly where the extender is implemented.
   - **Class** – the exact class that defines the extender's functionality (including any namespaces).
4. Click **Save**.

The extender code now runs when displaying any form containing a field with the given inherited form control.

### Example

The following example demonstrates how to create an extender for a form control inherited from the _Drop-down list_. The example sets the value of the form control's _options_ parameter (defines the options available in the selector) and performs custom validation for the value of the given form field.

Prepare a separate project for custom classes in your Xperience solution:

1. Open your Xperience solution in Visual Studio.
2. Create a new _Class Library_ project in the Xperience solution named **CustomExtenders**.
3. Add references to the required Xperience libraries (DLLs) for the new project:

   1. Right-click the project and select **Add -> Reference**.
   2. Click the **Assemblies -> Framework** tab in the **Reference manager** dialog and select the **System.Web** assembly.
   3. Switch to the **Browse** tab, click **Browse**, and navigate to the  _**Lib**_  folder of your Xperience administration project.
   4. Add references to the following libraries:

      - **CMS.Base.dll**
      - **CMS.Base.Web.UI.dll**
      - **CMS.Core.dll**
      - **CMS.FormEngine.dll**
      - **CMS.FormEngine.Web.UI.dll**
4. Reference the _CustomExtenders_ project from the Xperience project _(CMSApp_\*)\*.

Continue by adding the extender class:

1. Create a new class under the _CustomExtenders_ project, inheriting from **ControlExtender**:

   ```csharp

   using System;
   using System.Collections.Generic;

   using CMS.Base.Web.UI;
   using CMS.FormEngine.Web.UI;

   namespace CustomExtenders
   {
       public class CustomSelectorExtender : ControlExtender<FormEngineUserControl>
       {
           // Adds custom code that runs when the extended form control is loaded into a form
           public override void OnInit()
           {
               // Assigns a handler to the form control's initialization event
               Control.Init += Control_Init;

               // Sets the value of the form control's "options" parameter
               // This defines the options available in the selector
               var optionValues = new List<string>
               {
                   ";(select a value)",
                   "value1;Name 1",
                   "value2;Name 2",
                   "value3;Name 3"
               };
               Control.SetValue("options", String.Join(Environment.NewLine, optionValues));
           }

           // Handler method that runs during the initialization of the form control
           private void Control_Init(object sender, EventArgs e)
           {
               // Checks that the control is loaded within an Xperience form (BasicForm)
               if (Control.Form != null)
               {
                   // Assigns a handler to the validation event of the form containing the control
                   Control.Form.OnItemValidation += Form_OnItemValidation;
               }
           }

           // Handler method for the validation event of the related form
           private void Form_OnItemValidation(object sender, ref string errorMessage)
           {

               // Ensures that the custom validation only applies to fields using the extended form control
               // (the event is triggered for all items within the given form)
               if (sender == Control)
               {
                   // Displays a validation error message if the form control's value is empty
                   // (if the first '(select a value)' option is selected)
                   if (String.IsNullOrEmpty(Convert.ToString(Control.Value)))
                   {
                       errorMessage = "Please select a value.";
                   }
               }
           }
       }
   }

   ```
2. Save all changes and **Rebuild** your solution.

To apply the extender to the form control:

1. Open the **Administration interface** application and select the **Form controls** tab.
2. Edit the inherited form control.
3. On the **General** tab, select your extender class in the **Control extender** section:

   - **Assembly name**: CustomExtenders
   - **Class**: CustomExtenders.CustomSelectorExtender
4. Click **Save**.

If you [assign the inherited form control to a form field](https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls/assigning-form-controls-to-fields.md) and view the resulting form, the extender ensures sets the options available in the selector and adds custom validation that prevents users from saving an empty value (i.e. the _"(select a value)"_ option).