DropDownList数据绑定，默认显示请选择

            string str = "server='(local)';database='mytable';uid='sa';pwd='sa'";     //创建连接字串
            SqlConnection con = new SqlConnection(str)              //创建连接对象
            con.Open();                                       //打开连接
             string strsql = "SELECT ID,部门名称 FROM 公司部门";       //创建执行SQL语句
            SqlCommand cmd = new SqlCommand(strsql,con);//创建Command对象
            SqlDataReader rd = cmd.ExecuteReader();   //创建DataReader对象
            this.bumen.DataSource =rd;
            this.bumen.DataTextField = "部门名称";
           this.bumen.DataValueField = "ID";
           this.bumen.DataBind();
           this.bumen.Items.Insert(0, new ListItem("请选择", ""));