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;

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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