新建default页面：
html：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!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>无标题页</title>
<script language="javascript">
function isOver(sText,len)
{
var intlen=sText.value.length;
if (intlen>len)
{
alert("The content length must Less than or Equal "+len);
sText.focus();
sText.select();
}
}
</script>
</head>
<body>
<form id="Form1" method="post" runat="server">
<asp:TextBox id="txtName" style="Z-INDEX: 102; LEFT: 200px; POSITION: absolute; TOP: 104px" runat="server"
TextMode="MultiLine" Height="112px" Width="271px"></asp:TextBox>
</form>
</body>
</html>
cs：
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
this.txtName.Attributes.Add("onblur", "isOver(this,10);");
}
}