ClientSideCallback  used to avoid form Postback for each 
and every time .I have posted some sample code also try
which is use for Dropdown


aspx.cs
 private string _pricecallbackResult, 
_pricecallbackResultCAT, _pricecallbackResultClass;


 public string GetCallbackResult()
    {
        return _pricecallbackResult;
    }

    public void RaiseCallbackEvent(string eventArgument)
    {

       

        string[] val = eventArgument.Split(',');
        string id = val[0].ToString();
        string k = val[1].ToString();
        if (val[1] == "1")
        {
            //  _pricecallbackResult = 
objCommonBL.GetProgramNameForRebate();

        }
        else if (val[1] == "2")
        {
            
                    
                    _pricecallbackResult = 
objCommonBL.GetCategoryNameForRebate(Convert.ToInt32(id));
                

        }
        else if (val[1] == "3")

        {
                 _pricecallbackResult = 
objCommonBL.GetClassNameForRebate(Convert.ToInt32(id));
             
        }


    }


in Aspx Java Script
*********************


    function PriceClientCallbackError(result, context)
    {
      alert(result);
    }

 function PriceClientCallback(result, context)
        { 
         ClassClear();
          if (!result)
          {
              return;
          }
          
          var resultValue = result.split(','); 
          for (var i = 0; i < resultValue.length; ++i)
          {
            var val = resultValue[i].split('~~');
            for(var j=0;j<val.length; ++j)
            {
                var values = val[j].split('~');
                var option = document.createElement
("OPTION");
                option.value = values[0];
                option.innerHTML = values[1];     
                document.forms[0].ctl00$ContentPlaceHolder1
$frmRebate$ddlCategory.appendChild(option);
             }
        }
     }