Ajax基本示例一

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检测用户名是否存在：

前台代码：

<html xmlns="http://www.w3.org/1999/xhtml" ><head id="Head1" runat="server">    <title></title>    <script type="text/javascript">        var xmlhttp;        function createxml() {         if (window.ActiveXObject) {                var ieArr = ["Msxml2.XMLHTTP.8.0", "Msxml2.XMLHTTP.7.0", "Msxml2.XMLHTTP.6.0", "Msxml2.XMLHTTP.3.0", "Msxml2.XMLHTTP", "Microsoft.XMLHTTP"];                for (var i = 0; i < ieArr.length; i++) {                    try {                        xmlhttp = new ActiveXObject(ieArr[i]);                    }                    catch (e) { } //抛异常，直到找到合适的版本                }            }            else if (window.XMLHttpRequest) {            xmlhttp = new XMLHttpRequest();            }        }        function sendxml() {            var name = document.getElementById("Text1").value;            if (name.length == 0) {                document.getElementById("msg").innerHTML = "用户名不能为空！";                return;            }                          createxml();                xmlhttp.onreadystatechange = disxml;                xmlhttp.open("GET", "CheckName.aspx?name=" + name, true);                xmlhttp.send(null);              }        function disxml() {            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {                document.getElementById("msg").innerHTML = xmlhttp.responseText;            }        }    </script></head><body>    <form id="form1" runat="server">    <div>        <input id="Text1" type="text" onblur="sendxml()" /><div id="msg"></div>    </div>    </form></body></html>

后台代码：

    public class DBHelper    {        private SqlConnection sqlconn = null;        private SqlCommand sqlcmd = null;        private SqlDataReader sqlread = null;        private static DBHelper db = null;        private DBHelper()        {            sqlconn = new SqlConnection(ConfigurationManager.ConnectionStrings["sqlDB"].ToString());        }        /// <summary>        /// 单例        /// </summary>        /// <returns></returns>        public static DBHelper createInstance()        {            if (db == null)            {                db = new DBHelper();            }            return db;        } public bool Check(string name)        {            string sql =string.Format( "select count(*) from userinfo where username='{0}' ",name);            sqlcmd = new SqlCommand(sql,sqlconn);            try            {                sqlconn.Open();                return (int)sqlcmd.ExecuteScalar() > 0;            }            catch (Exception)            {                throw;            }            finally            {                sqlconn.Close();            }        }}

Checkname.aspx页面

        protected void Page_Load(object sender, EventArgs e)        {            if (!IsPostBack)            {                if (DBHelper.createInstance().Check(Request.QueryString["name"]))                {                    Response.Write("用户名已存在！");                    Response.End();                }                else                {                    Response.Write("用户名可用！");                    Response.End();                }            }        }