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();
    }