The Update Panel operates as something of a black-box, wrapping ASP.
NET postbacks within the context of a Java Script XMLHttp Request.
This control allows you to wrap markup which you would like to allow to be partially updated, that is, updated without causing a real postback to the server. Besides those two controls, everything else is standard controls, with no modifications that would indicate alternate behavior.
Try running the example site, and click the button.
NET AJAX Extensions, including Web Services access and JSON serialization - Java Script Object Notation) and includes an Update Panel and Content Template within the main Web Forms page by default.
Enabling partial rendering with this default page is as simple as revisiting Step 10 of this walkthrough and dropping controls onto the page.
An Update Panel can be added to any user control or custom control; however, the page on which these controls are included must also include a Script Manager control with the property Enable Partial Rendering set to true.
Markup-Enabled Properties: Quite simply, the above script registers a callback with the client-side AJAX runtime for when the asynchronous request has been completed.
It then checks to see whether an error was reported, and if so, processes the details of it, finally indicating to the runtime that the error was handled in custom script.
One way in which you might account for this when using Web Custom Controls is to override the protected class.
By doing so, you can inject an Update Panel between the control's children and the outside world if you determine the page supports partial rendering; otherwise, you can simply layer the child controls into a container instance.