Ajax 判断数据库表中该用户是否存在

index.html

var xmlHttp;

        //创建XMLHttpRequest对象
        //目的：兼容浏览器
        function createXMLHttpRequest() {
            if (window.ActiveXObject) {
                xmlHttp = new ActiveXObject("Microsoft.XMLHttp");
            }
            else if (window.XMLHttpRequest) {
                xmlHttp = new XMLHttpRequest();
            }
        }

function StringBuilder() {

var userName = document.getElementById("User").value;

return encodeURI(encodeURI(userName)); //两次编码解决中文乱码问题
}

　　 //当状态改变的时候

        function StateChange() {
            if (xmlHttp.readyState == 4 && xmlHttp.Status == 200) {
                var responseDiv = document.getElementById("back");
                responseDiv.innerHTML = decodeURI(xmlHttp.responseText); //解码
            }
        }

//发送

function SendToPage() {

createXMLHttpRequest();

var queryString = "Page_1.aspx?U=";
queryString += StringBuilder();

xmlHttp.open("GET", queryString);

xmlHttp.onreadystatechange = StateChange;

xmlHttp.send(null);
}

IsExists.aspx.cs

protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.QueryString["U"] != null)
            {
                Session["User"] = Request.QueryString["U"].ToString();
                Response.Write(Check());
                Response.End();
            }
        }

        protected string Check()
        {
            string sql = string.Empty;
            string res = string.Empty;

            if (Session["User"] != null)
            {
                sql = "select count(*) from T_Person where C_Name='" + Session["User"].ToString() + "'";
            }
            else
            {
                res = "error";
                return res;
            }

            if ((int)new SQLHelper().ExecuteScalar(sql) == 0)
            {
                res = "不存在该用户名!";
            }
            else
            {
                res = "已存在该用户名!";
            }
            return res;
        }

</script>