取dropdownlist 项的VALUE有关问题

取dropdownlist 项的VALUE问题
<asp:DropDownList   ID= "DropDownList2 "   runat= "server ">
                                                <asp:ListItem   Value= "idname "> 工号 </asp:ListItem>
                                                <asp:ListItem   Value= "name "> 姓名 </asp:ListItem>
                                        </asp:DropDownList> </td>
想取得 "工号 "的VALUE,做查询条件
尝试这样做不行,请指出错误地方:  
              select   *   from   email   where   "   +this.DropDownList2.SelectedValue.ToString();   +   "= ' "   +   TextBox1.Text.Trim()   +   " '

------解决方案--------------------------------------------------------
string sql
sql= "select * from email where " +this.DropDownList2.SelectedValue.ToString() + "= ' " + TextBox1.Text.Trim() + " ' ";

------解决方案--------------------------------------------------------
sql= "select * from email where 工号字段　＝　 ' " +this.DropDownList2.SelectedValue.ToString()+ " '＂