Please refer this code
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript">
function CheckTextBoxes(sender, args) {
var name = document.getElementById("<%=txtName.ClientID %>").value;
var city = document.getElementById("<%=txtCity.ClientID %>").value;
if (name == "" && city == "") {
args.IsValid = false;
sender.innerHTML = "Please Enter Name and City";
}
else if (name == "" && city != "") {
args.IsValid = false;
sender.innerHTML = "Please Enter Name";
}
else if (name != "" && city == "") {
args.IsValid = false;
sender.innerHTML = "Please Enter City";
}
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
Name
<asp:TextBox ID="txtName" runat="server" />
<br />
City
<asp:TextBox ID="txtCity" runat="server" />
<br />
<asp:CustomValidator ID="valValidateTextBox" runat="server" Display="Dynamic" ErrorMessage="Required"
ForeColor="Red" ClientValidationFunction="CheckTextBoxes" />
<br />
<asp:Button Text="Submit" runat="server" />
</div>
</form>
</body>
</html>
