when i refresh the page,dropdown value also refresh

OnSelectedIndexChanged="DDL_OSIC"

AutoPostBack="true"

EnableViewState="true">

 

Secondly make sure you are binding data to dropdownlist one time only. Use IsPostBack property to bind data when page loads first time. 

 

protected void Page_Load(object sender, EventArgs e)

{

   if (!IsPostBack)

   {

      DDL.Items.Add("red");

      DDL.Items.Add("green");

      DDL.Items.Add("blue");

   }

}