html5 验证 asp:dropdownlist

html5 validation on asp:dropdownlist

有没有办法在 asp:dropdownlist 上使用 html5 验证(而不是 asp:RequiredFieldValidator)。我已经试过了:

<asp:DropDownList ID="ExpenseLineTypeDdl" required="required" initialvalue="0" CssClass="form-control" runat="server">  
     <asp:ListItem Value="0">Please Select a Type</asp:ListItem>    
</asp:DropDownList>

我希望它的工作方式与此类似(asp:dropdownlist 除外):

<asp:TextBox runat="server" required="required"></asp:TextBox>

我不得不摆脱 {initialvalue="0"} 并设置初始列表项的 {value=""}:

<asp:DropDownList ID="ExpenseLineTypeDdl" required="required" CssClass="form-control" runat="server">  
      <asp:ListItem Value="">Please Select a Type</asp:ListItem>    
</asp:DropDownList>