JQuery ajax get returns null

Question

// THIS IS MY JAVA SCRIPT CODE

<script type="text/javascript">
        function getAllMessages() {
            $.ajax({
                url: "TestService.asmx/GetAllMessages",
                dataType: "json",
                method: "GET",
                contentType: "application/json",
                success: onSuccess,
                error: function (err) {
                    console.log(err);
                }
            });
        }

        function onSuccess(data) {
            console.log(data);
            var messageTable = $("#messages tbody");
            alert(data);
            $(data).each(function (index, msg) {
                var apnString = "<tr><td>" + msg.ID + "</td><td>" + msg.Email + "</td><td>" + msg.Message + "</td><td>" + msg.TimeStamp + "</td></tr>";
                messageTable.append(apnString);
            });
        }
    </script>

// WebMethod Code

 

SQLHelper clSQLHelper = new SQLHelper();

        [System.Web.Script.Services.ScriptMethod(UseHttpGet = true, ResponseFormat = System.Web.Script.Services.ResponseFormat.Json)]
        [WebMethod]
        public void GetAllMessages()
        {
            List<Messages> listMsg = new List<Messages>();
            DataTable dtMsg = clSQLHelper.PGSQLExecuteReader("get_all_messages", new List<NpgsqlParameter>(), "Messages");
            if (dtMsg != null && dtMsg.Rows.Count > 0)
            {

                foreach (DataRow dr in dtMsg.Rows)
                {
                    Messages Msg = new Messages();
                    Msg.ID = Convert.ToInt32(dr["auto_id"]);
                    Msg.Email = dr["email_id"].ToString();
                    Msg.Message = dr["message"].ToString();
                    Msg.TimeStamp = Convert.ToDateTime(dr["created_on"], new CultureInfo("en-IN"));
                    listMsg.Add(Msg);
                }

                JavaScriptSerializer js = new JavaScriptSerializer();
                Context.Response.Write(js.Serialize(listMsg));
            }
        }

 Inside success callbak function, data value is null.