如何让一个DropDownList绑定值后默认值为空

SqlConnection con = new SqlConnection("server=.;uid=sa;database=pubs");

SqlDataAdapter sdr = new SqlDataAdapter("select * from employee",con);

DataSet ds = new DataSet();

sdr.Fill(ds);

DropDownList1.DataSource = ds;

DropDownList1.DataTextField = "emp_id";

DropDownList1.DataBind();

DropDownList1.Items.Add(" ");

DropDownList1.Items[this.DropDownList1.Items.Count - 1].Value = " ";

DropDownList1.SelectedIndex = this.DropDownList1.Items.Count - 1;

-----------------------------------------------------------------

this.ddl_xmjl.Items.Insert(0, new ListItem("", "0"));  //单独使用，0代表位置

红色或者蓝色单独使用，都可以