The ListSearchExtender lets you search for items in a ListBox or DropDownList by typing. The extender performs an incremental search within the ListBox based on what has been typed so far.
the code is simple
<ajaxToolkit:ToolkitScriptManager ID="ScriptManager1" EnablePartialRendering="true" EnableScriptGlobalization="true" EnableScriptLocalization="true" runat="Server" />
ScriptManger is required for all AJAX controls..
<ajaxToolkit:ListSearchExtender ID="ListSearchExtender1" runat="server" TargetControlID="ListBox1" PromptCssClass="ListSearchExtenderPrompt" PromptPosition="Top"></ajaxToolkit:ListSearchExtender/>
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PromptText - Message to display when the ListBox is given focus.
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TargetControlID - Control to be searched
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PromptPosition - Indicates whether the message should appear at the Top or Bottom of the ListBox. The default is Top.