asp.net – RequiredFieldValidator不适用于Dropdownlist

<asp:DropDownList runat="server" ID="cboNoOfSubjects" AutoPostBack="true" CssClass="textEntry"  width="80%" onclick="javascript:shouldsubmit=false;" ValidationGroup="valCourseFees">
   <asp:ListItem Value="0">-Select-</asp:ListItem>
   <asp:ListItem Value="1">1</asp:ListItem>
   <asp:ListItem Value="2">2</asp:ListItem>
   <asp:ListItem Value="3">3</asp:ListItem>
   <asp:ListItem Value="4">4</asp:ListItem>
   <asp:ListItem Value="5">5</asp:ListItem>
</asp:DropDownList>
    <font color ="red">*</font> 

<asp:RequiredFieldValidator ID="cboNoOfSubjects_RequiredFieldValidator" 
 runat="server" ErrorMessage="No. of Subjects Required" ForeColor="Red"
 Font-Size="0.9em" ControlToValidate="cboNoOfSubjects" 
ValidationGroup="valCourseFees" Display="None"></asp:RequiredFieldValidator>

但验证器似乎没有发射.谁能帮我这个？

<asp:RequiredFieldValidator ID="cboNoOfSubjects_RequiredFieldValidator" runat="server"  
 ErrorMessage="No. of Subjects Required" ForeColor="Red" InitialValue="0"
 Font-Size="0.9em" ControlToValidate="cboNoOfSubjects" 
 ValidationGroup="valCourseFees" Display="None"></asp:RequiredFieldValidator>

还有一件事需要改变,你必须设置属性Display =“Dynamic”,它将相应地显示.