html>
    head>
        title>Ajax/title>
        script type="text/javascript">
            function loadXMLDoc() {
                if (document.getElementById("account").value == "") {
                    document.getElementById("accDiv").innerHTML = "用户名不能为空";
                    return;
                }
                var xmlHttp;
                if(window.XMLHttpRequest) { // code for IE7+
                    xmlHttp = new XMLHttpRequest();
                }
                else { // code for IE5/IE6
                    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
                }

                xmlHttp.onreadystatechange = function () {
                    if (xmlHttp.readyState == 4 xmlHttp.status == 200) {
                        //document.getElementById("myDiv").innerHTML=xmlHttp.responseText;
                        if (xmlHttp.responseText == "true") {
                            document.getElementById("accDiv").innerHTML = "用户名不可用";
                        }
                        else {
                            document.getElementById("accDiv").innerHTML = "用户名可用";
                        }
                    }
                }
                var a = document.getElementById("account").value;
                // get
                xmlHttp.open("GET", "validate.aspx?account=" + a + "random=" + Math.random, true);
                xmlHttp.send();
            }
            function delData() {
                document.getElementById("account").value = "";
                document.getElementById("accDiv").innerHTML = "";
            }
        /script>
    /head>
    body>
        h3>ajax/h3>
        table>
            tr>
                td>账号：/td>td>input id="account" type="text" onblur="loadXMLDoc();" onfocus="delData();"/>/td>td>div id="accDiv">/div>/td>
            /tr>
            tr>
                td>密码：/td>td>input id="passwd" type="password" />/td>
            /tr>
            tr>
                td>确认密码：/td>td>input id="vPasswd" type="password" />/td>
            /tr>
            tr>
                td>姓名：/td>td>input id="name" type="text" />/td>
            /tr>
        /table>

    /body>
/html>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Data.Sql;
using System.Data.SqlClient;

public partial class Ajax_validate_validate : System.Web.UI.Page
{
    public SqlConnection conn;

    protected void Page_Load(object sender, EventArgs e)
    {
        Response.Clear();
        if (Exists(Request.QueryString["account"]))
            Response.Write("true");
        else
            Response.Write("false");
        Response.End();
    }
    /// summary>
    /// 获取数据库连接
    /// /summary>
    /// returns>/returns>
    protected SqlConnection GetConnection()
    {
        string str = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        conn = new SqlConnection(str);
        return conn;
    }
    protected bool Exists(string account)
    {
        using (GetConnection())
        {
            try
            {
                conn.Open();
                string sqlStr = "select count(*) from userinfo where account='" + account + "'";
                SqlCommand cmd = new SqlCommand(sqlStr, conn);
                int row = Convert.ToInt32(cmd.ExecuteScalar());
                if (row > 0)
                    return true;
                else
                    return false;
            }
            catch (Exception e)
            {
                throw e;
            }
            finally
            {
                conn.Close();
            }
        }
    }
}