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):

 1 protected void Page_Load(object sender, EventArgs e) {

 2         if (!IsPostBack) {

 3             Response.Write("<script>alert('Is not PostBack')</script>");

 4             ddlstSinger.Items.Clear();

 5             adapter = new t_musicTableAdapter();

 6             var data = from d in adapter.GetData().AsQueryable()

 7                        group d by d.singer

 8                            into mySinger

 9                            select mySinger.First();

10             foreach (var d in data) {

11                 ddlstSinger.Items.Add(d.singer);

12             }

13         }

14     }

15     protected void btnSearch_Click(object sender, EventArgs e) {

16         if (ddlstSinger.SelectedItem == null) {

17             Alert("Please enter the search condition!");

18             return;

19         }

20         lbInvisible.Text = ddlstSinger.Text;

21         lvwSearch.DataSourceID = "ObjectDataSrcSearch";

22         lvwSearch.DataBind();

23     }