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Overview

Callback controls include Callback and CallbackPanel. Both use AJAX (Asynchronous JavaScript and XML) technology to communicate with server side function to update portion of the client-side page without reloading the whole page.

CallbackPanel control can contain other ASP.NET controls. Content inside the CallbackPanel can be updated from server "in live" and end user has a better experience.

Callback control cannot contain other ASP.NET controls. It works exactly the same as CallbackPanel except the partial page update function. This is useful when you need to send requests to backend server and do not wish to update any content on the page.

Specify Triggers property to specify which HTML or ASP.NET control's postback event will trigger the Callback. You do not need to write a single line of JavaScript to use Callback controls.

Rendering content through CallbackPanel

CallbackPanel control can be used as a container to hold the controls that will be updated when a Callback event happens. For example, in a product information page like the following:

When a certain product is selected by end user, CallbackPanel control can be used to pass this selected product ID to the server, retrieve information from backend database and update the product information area on the web page. End user will not experience the annoying refresh caused by the whole page being reloaded.
You can refer to Walkthrough: Rendering Content through CallbackPanel for detail steps.