<%@ Page Title=”" Language=”C#” AutoEventWireup=”true” CodeFile=”Default2.aspx.cs” Inherits=”Default2″ %>

<%@ Register Assembly=”AjaxControlToolkit” Namespace=”AjaxControlToolkit” TagPrefix=”cc1″ %>

<style type=”text/css”>

.watermark
{
color: #999999;
font-style: italic;
}
</style>

<asp:TextBox ID=”TextBox4” runat=”server”></asp:TextBox>

<cc1:TextBoxWatermarkExtender ID=”TextBoxWatermarkExtender1″ runat=”server” WatermarkCssClass=”watermark”

TargetControlID=”TextBox4”
WatermarkText=”Enter First Name“>
</cc1:TextBoxWatermarkExtender>

Using RegularExpressionValidator

<asp:TextBox ID=”TextBox1″ runat=”server”></asp:TextBox>

<asp:RegularExpressionValidator runat=”server” ControlToValidate=”TextBox1″

ErrorMessage=”Enter a valid number” ValidationExpression=“^\d$“>
</asp:RegularExpressionValidator>

using Javascript

<script type=”text/javascript”>

function isNumberKey(evt)

{

var charCode = (evt.which) ? evt.which : event.keyCode

if (charCode > 31 && (charCode < 48 || charCode > 57) )

return false;

return true;

}

</script>

protected void Page_Load(object sender, EventArgs e)

{

txtAdvance.Attributes.Add(”onkeypress”, “return isNumberKey(event)”);

}

Note: txtAdvance is Textbox ID Name

Above javascript from http://forums.asp.net/t/959255.aspx