IsPostBack and DropdownList.

Encounted the issue accident when helping my classmate dealing with his homework assignment,it turns out that I have to consider about 'IsPostBack' when trying to combind the 'DropdownList' in code behind(or it won't get what we expect):

 protected void Page_Load(object sender, EventArgs e) {
         if (!IsPostBack) {
             Response.Write("<script>alert('Is not PostBack')</script>");
             ddlstSinger.Items.Clear();
             adapter = new t_musicTableAdapter();
             var data = from d in adapter.GetData().AsQueryable()
                        group d by d.singer
                            into mySinger
                            select mySinger.First();
             foreach (var d in data) {
                 ddlstSinger.Items.Add(d.singer);
             }
         }
     }
     protected void btnSearch_Click(object sender, EventArgs e) {
         if (ddlstSinger.SelectedItem == null) {
             Alert("Please enter the search condition!");
             return;
         }
         lbInvisible.Text = ddlstSinger.Text;
         lvwSearch.DataSourceID = "ObjectDataSrcSearch";
         lvwSearch.DataBind();
     }