how can i stop this this event asp.net ?

Hi Abdalla Elawad,

Autopostback:

Autopostback is the mechanism by which the page will be posted back to the server automatically based on some events in the web controls. In some of the web controls, the property called auto post back, if set to true, will send the request to the server when an event happens in the control.

AutoPostBack = true permits control to post back to the server. It is associated with an Event.

aspx:

<asp:DropDownList id="id" runat="server" AutoPostBack="true" OnSelectIndexChanged="..."/>