dropDownList第一项为空

 1          SqlConnection conn = new SqlConnection("server=.;uid=sa;database=pubs");
 2          SqlDataAdapter dap = new SqlDataAdapter("select * from jobs", conn);
 3          DataTable dt = new DataTable();
 4          dap.Fill(dt);
 5          DropDownList1.Items.Clear();
 6          DropDownList1.DataSource = dt;
 7          DropDownList1.DataTextField = "job_desc";
 8          DropDownList1.DataValueField = "job_id";
 9          DropDownList1.DataBind();
          DropDownList1.Items.Insert(0, new ListItem("", ""));//插入空项，此举必须放到数据绑定之后