UpdateMarks.asmx ( LOCATION ROOT FOLDER)
Code:
using System;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Data.SqlClient;
using System.Data;

/// <summary>
/// Summary description for UpdateMarks
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]
public class UpdateMarks : System.Web.Services.WebService
{

    public SqlConnection con = new SqlConnection("Data source = SONIA-PC;Initial Catalog=master;Integrated Security=true");
    string query;
    SqlCommand cmd;
    DataSet ds = new DataSet();
    SqlDataAdapter da;

    [WebMethod]
    public string GetMarks(string Roll)
    {
        string Marks;
        cmd = new SqlCommand("Select Marks from student where Roll = '" + Roll + "'", con);
        Marks = cmd.ExecuteScalar().ToString();
        return Marks;
    }
}
ASPX PAGE
Code:
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Untitled Page</title>   </head>
<body>
    <form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate >
        <asp:DropDownList ID="cmbRoll" runat="server">
        </asp:DropDownList>
        <asp:TextBox ID="txtSubjectMarks" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        </ContentTemplate>
        </asp:UpdatePanel>
           </form>
    
     
</body>
&lt;script src =~/Common.js" type ="text/javascript" >&lt;/script&gt;
    &lt;script language="javascript" type ="text/javascript" >
function UpdateTotalMarks()
{
    alert("UpdateTotalMarks");
    var cmbRoll_Index = document.getElementById('cmbRoll').selectedIndex;
    alert(cmbRoll_Index);
    var cmbRoll_Text = document.getElementById('cmbRoll').options[cmbRoll_Index].text;
    alert(cmbRoll_Text);

    var validValue = GetSynchronousJSONResponse('<%=Page.ResolveUrl("~/UpdateMarks.asmx/GetMarks") %>', '{"Roll":"' + cmbRoll_Text + '"}');
    alert("a");
    correct = eval('(' + validValue + ')');
    if(isNaN(correct.d))
    {
        alert("0");
    } 
    else
    {
        document.getElementById('<%= TextBox1.ClientID %>').value = ConvertToPrecision(correct.d);
    }
}
    &lt;/script&gt;
</html>
SERVER SIDE CODE : -
Code:
protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            txtSubjectMarks.Attributes.Add("onKeyUp", "UpdateTotalMarks();");
Common.JS (LOCATION - Root FOLDER)
// JScript File

Code:
function GetSynchronousJSONResponse(url, postData) {
        var xmlhttp = null;
        if (window.XMLHttpRequest)
            xmlhttp = new XMLHttpRequest();
        else if (window.ActiveXObject) {
            if (new ActiveXObject("Microsoft.XMLHTTP"))
                xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            else
                xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
        }

        url = url + "?rnd=" + Math.random(); // to be ensure non-cached version

        xmlhttp.open("POST", url, false);
        xmlhttp.setRequestHeader("Content-Type", "application/json; charset=utf-8");
        xmlhttp.send(postData);
        var responseText = xmlhttp.responseText;
        return responseText;
    }
    
    
function ConvertToPrecision(value)
{
    return  Math.round (value *10000)/10000;
}
What i want to do?? I have two textboxes & One dropdownlist that contains suppose the roll nos. In one textbox I entered any number, as the number is entered, I want that marks of Roll (SELECTED FROM DROPDOWNLIST) are selected from the database + ADD the MArks entered in textbox.

ERROR - My webservice is not returning any value. I think so webservice is not called. I am not getting where the error?? PLEASE HELP ME OUT!!!