how to avoid page refresh on autopostback

Hello ,
ASP.Net is a server side.Every Code you write executes on the server side.The page gets refreshed with every event & the complete page cycle follows.
In order to avoid postback ,you have to use ajax.if you are not familair with ajax . you have to refer following link:
Ajax Link

Tutorail on ajax

Ajax for beginers

Hope it helps you

Right now in this scenario you can use Ispostback property of the page. and try not to hit the sql server in postbacks.

or if you don't want the page to sumbit to server then you have to use jquery or ajax.

To solve your problem, you must choose:
1. You can remove your code from page load or ensure it is not callet twice
2. Change the property of the dropdown list, so that it is not autopostback

But where to store the value you get then?
a. Session state (do you have session)
b. Page ViewState (do you have that)
c. Cookie (can you control if your users enable cookies?)

Essentially the challenges are that (a.) if your site maintains session state you will be able to serve significantly less traffic/ second. If you use ViewState (b.) your page roundtrips will carry a significantly bigger payload which will have to be sent to and from the server and (c.) cookies can be disabled.
So say we choose (a.) for persistance we have to solve(1.) the problem that every load will refresh your textbox value, you can make a session based member property to store it with. Presume you cannot change the autopostback mode of the dropdown. But I greatly recommend that you disable page viewstate, though if you had had it you could simply use the ispostback to only load controls on initial get.

protected string SerialNo{
  get{
      string serial = Session["serialno"];
      if(string.isnullorempty(serial)){
         serial = GetNewSerialNumber();
         Session["serialno"] = serial;
      }
      return serial;
    }
}

Page_Load(object sender, eventargs e){
    MySerialNoTextBox.Text = SerialNo;
    if(!IsPostback){
      //Alternatively just load your serial number inhere, but that would require viewstate
      /*
      MySerialNoTextBox.Text = GetNewSerialNumber();
      */
    }
}

Hope this helps