<asp:DropDownList ID="ddlCountry" runat="server">
    <asp:ListItem Value="">-- 请选择国家 --</asp:ListItem>
    <asp:ListItem Value="1">中国</asp:ListItem>
    <asp:ListItem Value="2">美国</asp:ListItem>
    <asp:ListItem Value="3">英国</asp:ListItem>
</asp:DropDownList>

<asp:RequiredFieldValidator ID="rfvCountry" runat="server" 
    ControlToValidate="ddlCountry"
    InitialValue=""
    ErrorMessage="请选择国家"
    Display="Dynamic"
    ForeColor="Red">
</asp:RequiredFieldValidator>

<asp:RequiredFieldValidator InitialValue="-1" ... />

<asp:DropDownList ID="ddlCity" runat="server" ValidationGroup="UserInfo">
    <!-- 列表项 -->
</asp:DropDownList>

<asp:RequiredFieldValidator ID="rfvCity" runat="server" 
    ControlToValidate="ddlCity"
    ValidationGroup="UserInfo"
    ...>
</asp:RequiredFieldValidator>

<asp:CustomValidator ID="cvCountry" runat="server" 
    ControlToValidate="ddlCountry"
    OnServerValidate="ValidateCountry"
    ClientValidationFunction="clientValidateCountry"
    ErrorMessage="无效的国家选择"
    Display="Dynamic">
</asp:CustomValidator>

protected void ValidateCountry(object source, ServerValidateEventArgs args)
{
    args.IsValid = ddlCountry.SelectedValue != "0"; // 自定义验证逻辑
}

function clientValidateCountry(sender, args) {
    args.IsValid = document.getElementById('<%= ddlCountry.ClientID %>').value != "0";
}