Ajax检测注册用户是否存在(.NET) - 编程论坛

HTML代码如下:
LoginValidate.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="LoginValidate.aspx.cs" Inherits="LoginValidate" %>

<html xmlns="http://www. >
<head runat="server">
<title>验证用户名是否存在</title>
<script type="text/javascript">
    var xmlHttp;
    function createXMLHttpRequest()
    {
        if(window.ActiveXObject)
        {
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        else if(window.XMLHttpRequest)
        {
            xmlHttp = new XMLHttpRequest();
        }
    }
    //处理方法
    function CheckUserName()
    {
        createXMLHttpRequest();
        var url= "LoginValidate.ashx?username="+document.getElementById("username").value;
        xmlHttp.open("GET",url,true);
        xmlHttp.onreadystatechange=ShowResult;
        xmlHttp.send(null);
        //document.getElementById("Msg").innerHTML='';
    }
    //回调方法
    function ShowResult()
    {
        if(xmlHttp.readyState==4)
        {
            if(xmlHttp.status==200)
            {
                document.getElementById("Msg").innerHTML=xmlHttp.responseText;
            }
        }
    }
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<table style="width: 487px">
    <tr>
        <td style="width: 70px">
            用户名：</td>
        <td style="width: 231px"><input id="username" type="text" />
        <input id="Button1" type="button" value="button" onclick="CheckUserName();" /></td>
        <td id="Msg"></td>
    </tr>
    <tr>
        <td style="width: 70px">
        </td>
        <td style="width: 231px">
        </td>
        <td>
            </td>
    </tr>
</table>
</div>
</form>
</body>
</html>

服务器端代码如下:(这里我是用的临时处理文件.ashx)
LoginValidate.ashx
<%@ WebHandler Language="C#" Class="LoginValidate" %>

using System;
using System.Web;
using System.Data;
using System.Data.SqlClient;

public class LoginValidate : IHttpHandler
{

    public void ProcessRequest (HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        string username = context.Request.QueryString["username"].ToString();
        string strSQL = "select username from users where username='" + username + "'";
        if (ReDataSet(strSQL).Tables[0].Rows.Count > 0)
        {
            context.Response.Write("该用户已经有人使用！");
        }
        else
        {
            context.Response.Write("恭喜你！"+username+"可以使用！");
        }
        System.Threading.Thread.Sleep(3000);
    }

    //数据库连接字符串
    public static string strCon = "Data Source=.;database=exam;uid=sa;pwd=;";

    /// <summary>
    /// 执行SQL语句，返回DataSet
    /// </summary>
    /// <param name="strSQL"></param>
    /// <returns></returns>
    public  DataSet ReDataSet(string strSQL)
    {
        SqlConnection con = new SqlConnection(strCon);
        try
        {
            con.Open();
            SqlDataAdapter da = new SqlDataAdapter(strSQL, con);
            DataSet ds = new DataSet();
            da.Fill(ds);
            return ds;
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }
        finally
        {
            con.Close();
        }
    }

    /// <summary>
    /// 不重复调用
    /// </summary>
    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}