In this blog you will learn that how to validate an control with popup using ajax:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="validatoreg.aspx.cs" Inherits="validatoreg" %>
importing ajaxcontrol toolkit
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>
Name: <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender1" TargetControlID="RequiredFieldValidator1" runat="server">
</asp:ValidatorCalloutExtender>
<asp:RequiredFieldValidator Display="None" ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1" ErrorMessage="User Name cannt be left blank"></asp:RequiredFieldValidator>
Email:<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender2" TargetControlID="RequiredFieldValidator2" runat="server">
</asp:ValidatorCalloutExtender>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" Display="None" runat="server" ControlToValidate="TextBox2" ErrorMessage="Email cannt be left blank"></asp:RequiredFieldValidator>
<asp:Button ID="b" runat="server" Text="Login" />
</form>
</body>
</html>
Thanks