We have already known that an extender’s goal is to wire a client component to an existing server control. Therefore, we have to make clear how the client functionality is attached to the extended control.

The easiest way to build an extender is to declare a class that inherits from the base ExtenderControl class. This class implements an interface called IExtenderControl and takes care of registering the extender with the ScriptManager control. A derived class should override the methods defined in the IExtenderControl interface. Let us look more closely into this interface before developing our first extender.

The IExtenderControl Interface and Registering the Extender

The IExtenderControl interface defines the contract to which a class adheres to become an extender. Listing 3 shows the methods implemented by the interface and Table 1 gives the detailed explanations.

Listing 3

public interface IExtenderControl
{
  IEnumerable<ScriptDescriptor> GetScriptDescriptors(Control targetControl);
  IEnumerable<ScriptReference> GetScriptReferences();
}

Table 1

The process of registering with the ScriptManager lets the extender be recognized as an Ajax-enabled control. Concretely, this is a two-step process:

1 During the PreRender stage, we will call RegisterExtenderControl method, passing the extender instance and the extended control as arguments.

2 During the Render stage, we will call the RegisterScriptDescriptors method to register the script descriptors.

The first part of the registration procedure involves calling the RegisterExtenderControl method on the ScriptManager control. This method receives the current extender instance and the extended control as arguments. The registration procedure is completed during the Render phase, where you call the RegisterScriptDescriptors method on the ScriptManager, passing the current extender instance as an argument.

Happily, the ExtenderControl class takes care of performing the registration procedure automatically.  Since we always create a new extender by deriving from the ExtenderControl class, we do not need to worry about the implementation details.  As for script controls to be discussed later, however, we will discover that in some situations we need to manually register the Ajax-enabled control with the ScriptManager.

In the following section, we will create an extender for the MyHoverBehavior behavior we built in earlier article "Explorer Ways to extend ASP.NET AJAX Client-side Function."  This will let you wire the behavior to an ASP.NET Panel control (you can of course try other types of controls) and configure it on the server side.