How to: Respond to Changes in List Web Server Controls
When the user makes a selection in a list Web server control, the control raises an event that you can respond to directly. Normally, you do not need to respond directly to the selection event at all. Instead, it is more typical to test which item is selected after the form has been posted to the server by a control such as the Button control. For details about determining which item is selected, see Determining the Selection in a List Web Server Control.
To respond directly to changes in a list Web server control
Create an event handler for the control's SelectedIndexChanged event. By default, the SelectedIndexChanged event does not immediately cause the page to be posted to the server. Instead, the event is raised in server code the next time the form is posted. To have the SelectedIndexChanged event cause an immediate posting, set the control's AutoPostBack property to true.
The event is raised as soon as the user makes a selection in the list control. If the AutoPostBack property is true, the form is posted with each selection, but the selected items are preserved with each round trip.
Controls in an ASP.NET page can include potentially malicious client script. By default, the Web Forms page validates that user input does not include script or HTML elements. For more information, see How to: Protect Against Script Exploits in a Web Application by Applying HTML Encoding to Strings.