Ajax post metod in C#

Place the following code in pages 

In aspx page
-----------------
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript" src="http://cdn.jsdelivr.net/json2/0.1/json2.js"></script>
<script type="text/javascript">
    $(function () {
        $("[id*=btnSubmit]").bind("click", function () {
            var user = {};
            user.Category=$("[id*=ddlCategory]").val();
            user.ID=getParameterByName('id');          
            user.Username = $("[id*=ddlCategory]").val();
            user.Password = $("[id*=txtPassword]").val();
            $.ajax({
                type: "POST",
                url: "Details.aspx/UpdateCategory",
                data: '{user: ' + JSON.stringify(user) + '}',
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (response) {
                    alert("Category Updated successfully.");
                    //window.location.reload();
                }
            });
            return false;
        });
    });
   
    function getParameterByName(name) {
    name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
    var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
        results = regex.exec(location.search);
    return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}
</script>

 <asp:DropDownList ID="ddlCategory" CssClass="smallfont" runat="server" DataTextField="Cat"
                                            DataValueField="HowMany">
                                        </asp:DropDownList><asp:Button ID="btnSubmit" runat="server" Text="Update" />
                                       

                           
In .cs file
--------------
[WebMethod]
        [ScriptMethod]
        public static void UpdateCategory(User user)
        {          
           //Code here            }
        }
        public class User
        {
            public string Category { get; set; }
            public int ID { get; set; }
        }