combobox绑定数据后添加一行

SqlDataAdapter da = new SqlDataAdapter("select distinct WorName,WorId from Workers ", cn);
            DataSet ds = new DataSet();           
            da.Fill(ds,"Workers");
            DataRow dr = ds.Tables["Workers"].NewRow();
            dr["WorId"] = "0";
            dr["WorName"] = "-请选择-";
            ds.Tables["Workers"].Rows.InsertAt(dr, 0);
            comboBox1.DataSource = ds.Tables["Workers"];
            comboBox1.DisplayMember = "WorName";
            comboBox1.ValueMember = "WorId";
            cn.Close();