Get Latitude and Longitude value of marker in GoogleMap and assign to Label using JavaScript

but i cant get that value on Double latitude,Double Longitude

Double latitude = Convert.ToDouble(lat.Value);
Double longitude = Convert.ToDouble(lng.Value);

This is the page full

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default5.aspx.cs" Inherits="Default5" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
        <script src="//maps.googleapis.com/maps/api/js?v=3.exp&sensor=false&libraries=places&key=AIzaSyBuPeoxlUbkNOl6IyUfnqx65cMfof7gcqo" async="" defer="defer" type="text/javascript"></script>
<script type="text/javascript">
    var long = 0;
    var lat = 0;
    window.onload = function () {
        var mapOptions = {
            center: new google.maps.LatLng(21.0000, 78.0000),
            zoom: 5,
            mapTypeId: google.maps.MapTypeId.ROADMAP

        };
        var map = new google.maps.Map(document.getElementById("dvMap"), mapOptions);

        //Attach click event handler to the map.
        google.maps.event.addListener(map, 'click', function (e) {

            //Determine the location where the user has clicked.
            var location = e.latLng;

            //Create a marker and placed it on the map.
            var marker = new google.maps.Marker({
                position: location,
                map: map

            });

            //Attach click event handler to the marker.
            google.maps.event.addListener(marker, "click", function (e) {

                var infoWindow = new google.maps.InfoWindow({
                    content: 'Latitude: ' + location.lat() + '<br />Longitude: ' + location.lng()

                });
                infoWindow.open(map, marker);

            });
            google.maps.event.addListener(map, 'click', function (e) {

                long = e.latLng.lng();
                lat = e.latLng.lat();
                document.getElementById("lat").value = lat;
                document.getElementById("lng").value = long;

                document.getElementById("<%=lat.ClientID%>").value = lat;
                document.getElementById("<%=lng.ClientID%>").value = long;                

                
                alert("Latitude: " + lat + "\r\nLongitude: " + long);
            });


        });
    };
</script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">

     <asp:HiddenField ID="lat" runat="server" />
   <asp:HiddenField ID="lng" runat="server" />
 <div id="dvMap" style="width: 500px; height: 500px">
 </div>

<asp:Button ID="btn" runat="server" Text="go" OnClick="btn_Click" />
</asp:Content>

 This is the code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient; 
public partial class Default5 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btn_Click(object sender, EventArgs e)
    {
        Double latitude = Convert.ToDouble(lat.Value);
        Double longitude = Convert.ToDouble(lng.Value);

        SqlConnection con = new SqlConnection();
        con.ConnectionString = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=F:\shafeek\aspprojects\WebSite1\App_Data\Database.mdf;Integrated Security=True";

        string query1 = "insert into Courses(longi,lati) values (@lati, @longi)";

        SqlCommand cmd1 = new SqlCommand(query1, con);
        cmd1.Parameters.AddWithValue("@lati", latitude);
        cmd1.Parameters.AddWithValue("@longi", longitude);

        con.Open();
        cmd1.ExecuteNonQuery();
        con.Close();
    }
}