Calculate Latitude and longitude using address in asp.net using Google Maps API V3

I want to caclate Latitude and Longitude from address in asp.net. I have written the necessary code ,but could not get the desired result

 I have put an alert and tested it.Its working fine until geocoder.geocode()function is called.Alert put inside the function is not working.It seems there is some issue with function.

This is my javascript function

function calculateCoordinates() {

    var txtAddress1 = document.getElementById('<%= txtAddress1.ClientID%>');
    var txtLatitude = document.getElementById('<%= txtLat.ClientID%');
    var txtLongitude = document.getElementById('<%= txtLong.ClientID%>');
    var address = txtAddress1.value; 
    var geocoder;
    geocoder = new google.maps.Geocoder();
    geocoder.geocode({ 'address': address }, function (results, status) {
        if (status == google.maps.GeocoderStatus.OK) {
            var location = results[0].geometry.location;
            txtLatitude.value = location.lat();
            txtLongitude.value = location.lng();
        }
        else
            alert(searchString + ' - not found');

    });

}

</script>