JsonConvert - Can not assign string value to a string variable

Question

public void FindCityName()
        {
            string url = "http://maps.google.com/maps/geo?q=39.920794,32.853902&output=json&oe=utf8&sensor=true&key=MYKEY";

            var w = new WebClient();
            Observable.FromEvent<DownloadStringCompletedEventArgs>(w, "DownloadStringCompleted").Subscribe(r =>
                {
                    var deserialized = JsonConvert.DeserializeObject<RootObject>(r.EventArgs.Result);


                    string s = deserialized.Placemark[0].AddressDetails.Country.SubAdministrativeArea.Locality.LocalityName;
/// setCity() and City=s produce the same thing
                    setCity(s);
                    City = s;
                    //foreach (var item in deserialized.Placemark)
                    //{
                    //    //MessageBox.Show(item.AddressDetails.Country.SubAdministrativeArea.Locality.LocalityName);
                    //    City = (string)item.AddressDetails.Country.SubAdministrativeArea.Locality.LocalityName;
                    //}
//Problem here >>>>>
                    ////MessageBox.Show(City);
                });
            w.DownloadStringAsync(new Uri(url)); 
        }

Problem:

I am working on a windows phone 7 application and I need to find the "City Name" from GPS coordinates in order to move forward...

I found the code above on the internet and tried it. I can see the city name by using these codes(Message.Box(City) show exactly what I want, the city name). However, this line of code

deserialized.Placemark[0].AddressDetails.Country.SubAdministrativeArea.Locality.LocalityName;

which gives me the city name seems to give a volatile string value.

For example, I created a method which assigns the value of string variable "s" to the string field of my class, name City. If I try to get the City's content after calling FindCityName() method, I see that City's content is not updated.

Again, same thing happens then I call the code line under the comment "Problem here >>>>>" that MessageBox.Show(City) shows nothing new...

Can someone explain me the reason of my problem?

Answer 1

you put this question on my blog as well, but I will answer it here. I feel a bit responsible for putting up the sample code in the first place ;-)

I am going to assume the class containing your code looks like this:

public class MyClass
{
    private void MyMethod()
    {
      FindCityName();
      MessageBox.Show(City);
    }

    private void FindCityName()
    {
      // Code omitted - see your question
    }

    private string City;
}

There is nothing volatile about the string. Your problem is asynchronicity. If you look carefully you will see that I use an observable that fires when the DownloadStringCompleted is fired. The code inside Observable.Event is only called when the download is finished but that happens asynchronously. But what I assume you do is call the FindCityName method and then directly trying to access results like I show in the MyMethod method. That's like directly wanting the result after firing the request. The results are not in yet! It's like a web page downloading - it takes a while. You can fix that with a callback, something like this:

public class MyClass
{
    private void MyMethod()
    {
      FindName();       
    }

    public void FindCityName()
    {
        string url = "http://maps.google.com/maps/geo?q=39.920794,32.853902&output=json&oe=utf8&sensor=true&key=MYKEY";

        var w = new WebClient();
        Observable.FromEvent<DownloadStringCompletedEventArgs>(w, "DownloadStringCompleted").Subscribe(r =>
            {
                var deserialized = JsonConvert.DeserializeObject<RootObject>(r.EventArgs.Result);

                City = deserialized.Placemark[0].AddressDetails.Country.SubAdministrativeArea.Locality.LocalityName;
                DoneDownloading();
            });
        w.DownloadStringAsync(new Uri(url)); 
    }

    private string City;

    private void DoneDownloading
    {
        MessageBox.Show(City);
    }
}

Does that help?

Answer 2

I would recommend you to use this Google Map API

http://maps.googleapis.com/maps/api/geocode/json?latlng=39.920794,32.853902&sensor=true

And once you get JSON response in your request. You can parse easily with NEWTONSOFT for wp7

WebClient wc = new WebClient();
var json = (JObject)JsonConvert.DeserializeObject(wc.DownloadString(url));

var locality= json["results"]
                .SelectMany(x => x["address_components"])
                .FirstOrDefault(t => t["types"].First().ToString() == "locality");

var name = locality!=null ? locality["long_name"].ToString() : "";