The UpdatePanel Control
The UpdatePanel is an ASP.NET server control that updates portions of a web page without reloading it. Through a mechanism called asynchronous postback, the HTML for the region of the page wrapped by the control is sent by the server asynchronously through an Ajax request.
The ASP.NET controls that have been specified as content in an UpdatePanel are able to cause either synchronous (traditional) or asynchronous postbacks, by means of triggers.
A trigger is an event coming from an ASP.NET control that causes an UpdatePanel to refresh its contents. Through triggers, an asynchronous postback can be started also by controls that are declared outside the region of the ASP.NET page wrapped by the UpdatePanel control.
In the following code, only the content of the Update control (the span element that displays the current date and time) is re-rendered every time the button is clicked.
Read more about this topic: ASP.NET AJAX
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