xmlhttp调用后台

    <script language="javascript" type="text/javascript">
         
        function CallServer(url) {

//            xml = new ActiveXObject("Microsoft.XMLHTTP");
//            var post = "";
//            xml.open("POST", url, false);
//            xml.setrequestheader("content-length", post.length);
//            xml.setrequestheader("content-type", "application/x-www-form-urlencoded");
//            xml.send(post);
//            var res = unescape(xml.responseText);
//            return res

            xmlHttp = GetXmlHttpObject();
            if (!xmlHttp || xmlHttp == null) {
                alert('browser does not support http request')
                return;
            }
            xmlHttp.open("GET", url, false);
            xmlHttp.send(null);
            var res = unescape(xmlHttp.responseText);
            return res
        }

        function CallServerAsy(url, ID) 
        {
            xmlHttp = GetXmlHttpObject();
            if (!xmlHttp || xmlHttp == null) {
                alert('browser does not support http request')
                return;
            }

           //xmlHttp.onreadystatechange = stateChanged;
           //xmlHttp.onreadystatechange = stateChanged(ID);
            xmlHttp.onreadystatechange =  function() { stateChanged(ID) }; 
            xmlHttp.open("GET", url, true);
            xmlHttp.send(null);
          
        }
    
    
        var xmlHttp;
        function showHint(str) 
        {
            if (str.length == 0) {
                document.getElementById("TextBoxHint").innerHTML = "";
                return;
            }

            var url = "LastNameLookup.aspx"
            url = url + "?q" + str;

            document.getElementById("TextBoxHint").innerHTML = "正在处理……";

            //同步调用
            //document.getElementById("TextBoxHint").innerHTML = CallServer(url);
            
            //异步调用
            CallServerAsy(url, "TextBoxHint")
        }

        function stateChanged(ID) 
        {
            var OK = 200;
            if ((xmlHttp.readyState == 4 || xmlHttp.readyState == "complete")
                && xmlHttp.status == OK) {      
                document.getElementById(ID).innerHTML = xmlHttp.responseText;            
            }
        }

        function GetXmlHttpObject(hander) {
            var objXMLHttp = null;
            if (window.XMLHttpRequest) {
                try {
              
                   objXMLHttp = new XMLHttpRequest();
                }
                catch(e)
                {
                }
            }
            else if(window.ActiveXObject)
            {
                try {
                   
                   objXMLHttp = new ActiveXObject("Microsoft.XMLHTTP");
                   
               }
               catch(e)
               {
               }
            }
            return objXMLHttp;
        }
    </script>
 

代码
<div>
       <h1>Lookup</h1>
       
       <b>Last name:</b><asp:TextBox ID="TextBox1" runat="server" onkeyup="showHint(this.value)"></asp:TextBox>
       
       <p><b><i>Names: </i></b>&nbsp;<span id="TextBoxHint"></span></p>
       
    </div>
 
LastNameLookup.aspx后台：
 
代码
                System.Threading.Thread.Sleep(10000);
                string queryLastName = Request.QueryString.Get(0).ToString();
                string returnString = "";
                returnString = "<select size=1>";
                returnString += "<option>111</option>";
                returnString += "<option>222</option>";
                returnString += "<option>333</option>";
                returnString += "<option>444</option>";
                returnString += "<option>555</option>";
                returnString += "</select>";
                Response.Write(returnString);
                Response.End();
 
注意点：open方法指定get还是post，同步还是异步，异步回调方法的指定和参数传递
       后台的Response.End(),否则会把所有网页都返回