<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default11.aspx.cs" Inherits="Default11" %>
<!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>
<script type = "text/javascript">
function ValidateListBox(sender, args) {
var options = document.getElementById("<%=ListBox1.ClientID%>").options;
for (var i = 0; i < options.length; i++) {
if (options[i].selected == true) {
args.IsValid = true;
return;
}
}
args.IsValid = false;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ListBox ID="ListBox1" runat="server" SelectionMode = "Multiple">
<asp:ListItem Text = "Apple" Value = "1"></asp:ListItem>
<asp:ListItem Text = "Mango" Value = "2"></asp:ListItem>
<asp:ListItem Text = "Orange" Value = "3"></asp:ListItem>
</asp:ListBox>
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="*Required"
ClientValidationFunction = "ValidateListBox"></asp:CustomValidator>
<asp:Button ID="Button1" runat="server" Text="Button" />
</div>
</form>
</body>
</html>
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