Ajax无刷新验证用户名是否注册过

JS脚步：

var req;
function validate()
{
var username=document.getElementById("txtStuNO").value;
if ((username == null) || (username == ""))
return;
var url="SValidate.aspx?userid=" + escape(username);
if(window.XMLHttpRequest)
{
req=new XMLHttpRequest();
}
else if(window.ActiveXobject)
{
req=new ActiveXObject("microsoft.XMLHTTP");
}
req.open("Get",url,true);
req.onreadystatechange=callback;
req.send(null);
}
function callback()
{
if(req.readystate<4)
document.getElementById("lblCheckName").innerText="验证用户...";
if(req.readystate==4)
if(req.status==200)
{
document.getElementById("lblCheckName").innerText=req.responseText;
}
}

验证页面

SValidate.aspx

cs文件代码：

ConDB condb = new ConDB();

protected void Page_Load(object sender, EventArgs e)
{
string userid = Request.QueryString["userid"];
string select = "select count(*) from tb_Student where StuNo='" + userid + "' ";

if (condb.SelSQL(select) == true)
{
Response.Write("用户名存在");
}
else
Response.Write("用户名可用");
}