ASP.NET Validator doesn't fire - asp.net

Question

I've a problem with a RequiredFieldValidator on a page with two Panels, which represent two different views, i.e. only one panel is visible at one time, but they share same submit button.
By default the "BatchReturnPanel" is visible and "SingleReturnPanel" is hidden, and in this case all validators on page work just fine. But if I hide the "batch.." panel and show "single.." panel (ChangeViewButton click event) then the requieredFieldValidator "DropDownListFieldValidator" doesn't fire up, and page is always considered validated. Why does this happen and how can this problem be solved? Thanks for help/
Below is extract from the ASPX page:
<div>
<asp:DropDownList ID="MDDropDownList" runat="server"></asp:DropDownList>
<asp:RequiredFieldValidator id="DropDownListFieldValidator" runat="server" ControlToValidate="MDDropDownList"
InitialValue="-" ErrorMessage="Please select Master Distributor" EnableClientScript="true" CssClass="error"
Display="Static" ValidationGroup="Voucher" />
<asp:Panel runat="server" ID="BatchReturnPanel">
<p>
<asp:TextBox ID="FirstVoucherTextBox" runat="server" CssClass="Scanner RRS"></asp:TextBox>
<asp:RequiredFieldValidator ID="FirstVoucherFieldValidator" runat="server" ControlToValidate="FirstVoucherTextBox" ValidationGroup="Voucher"
ErrorMessage="Valid Voucher ID for the first voucher in batch is required<br>" Display="Dynamic" CssClass="error" EnableClientScript="false"></asp:RequiredFieldValidator>
</p>
</asp:Panel>
<asp:Panel runat="server" ID="SingleReturnPanel" Visible="false">
<p>
<asp:TextBox ID="SingleVoucherTextBox1" runat="server"></asp:TextBox><br />
<asp:TextBox ID="SingleVoucherTextBox2" runat="server"></asp:TextBox><br />
<asp:TextBox ID="SingleVoucherTextBox3" runat="server"></asp:TextBox><br />
</p>
</asp:Panel>
<asp:Button ID="VoucherSubmitButton" runat="server" Text="Enter"
OnClick="VoucherSubmitButton_Click" UseSubmitBehavior="false" ValidationGroup="Voucher"/>
</div>
<asp:Panel CssClass="grid_4 box" runat="server" ID="ChangeViewPanel">
<asp:LinkButton runat="server" ID="ChangeViewButton"
onclick="ChangeViewButton_Click">Single item return mode</asp:LinkButton>
</asp:Panel>

Answer 1

You can use validation group and client side page validation.
function Validation() {
Page_ClientValidate('validationGroupName');
if (!Page_IsValid) {
return false;
}
else {
return true;
}
}
Hope this helps