Here I am gonna use JavaScript and CSS to display a popup-style error window for validation like this:

It still uses all validators the same as in Part One, but with some JavaScript functions and some CSS tricks, we can make it look like a pop up.

Here is the code:

Submit button:

<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClientClick="validateInput()" />

ValidationSummary control:

<asp:ValidationSummary ID="valSummary" runat="server" DisplayMode="List" />

Popup panel:

<asp:Panel ID="pnlPopupError" runat="server" Width="250px"
CssClass="PopupBackground" style="display:none;">
<div class="Popup">
<div id="divError"></div>
<input type="button" value="OK" >
JavaScript code:
CSS:
Happy programming!