AJAX判断用户名，方法未执行,该怎么解决

var xmlHttp;        function createXmlHttpRequest() {            if (window.XMLHttpRequest) {                xmlHttp = new XMLHttpRequest();                if (xmlHttp.overrideMimeType) {                    xmlHttp.overrideMimeType("text/xml");                }            }            else if (window.ActiveXObject) {                try {                    xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");                }                catch (e) {                    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");                }            }            if (!xmlHttp) {                window.alert("你的浏览器不支持创建XMLhttpRequest对象");            }            return xmlHttp;        }function checkname(name) {            createXmlHttpRequest();            var url = "Customer/createcustomerAJAX.aspx?Name=" + name;            xmlHttp.open("post", url, true);            xmlHttp.onreadystatechange = CheckUserNameResult;            xmlHttp.send(null);        }        function CheckUserNameResult() {            if (xmlHttp.readyState == 4)//服务器响应状态            {                if (xmlHttp.status == 200)//代码执行状态                {                    if (xmlHttp.responseText == "true") {                        document.getElementById("img_companyname").src = "../images/正确 .jpg";                    }                    else {                        document.getElementById("imgflag").src = "../images/不能为空.jpg";                        document.getElementById("Text1").style.display = true;                    }                }            }        }

checkcompanyname cu = new checkcompanyname();            if (cu.checkcompany(Request.QueryString["Name"].ToString()))            {                Response.Write("false");                Response.End();            }            else            {                Response.Write("true");                Response.End();            }

public bool checkcompany(string name)        {            string sql = "select * from customers where companyname='"+name+"'";            SqlConnection con = new SqlConnection(SqlHelper.connectionString);            con.Open();            SqlCommand com = new SqlCommand(sql,con);            SqlDataReader dr = com.ExecuteReader();            if (dr.Read())            {                return true;            }            else            {                return false;            }        }