Working with JSON web services in Silverlight and Windows Phone 7

In this post I’ll explain how to retrieve and display data from JSON web services.

As an example I’ll use the REST service from geonames.org.

JSON response of the example looks so:

At first, you should create C# classes (models) which can be mapped to JSON entities.
The first curly braсe indicates that it is an object, and it has 1 property `geonames`. After this property you can see the square bracket indicating an array. This array consists of many objects, and each object has 12 properties.
So we should create a model for the root object, which looks so:

// { geonames: [{}, {}, {}] }
[DataContract]
public class CitiesList
{
	[DataMember(Name = "geonames")]
	public List<City> Cities { get; set; }
}

It is important to add DataContract and DataMember attributes to the class and all its properties. Also you should specify the name of the corresponding JSON property (geonames) inside the DataMember attribute.

Then we should create the model for inner objects (cities). As you already saw, each JSON object has 12 properties, but we don’t need all of them. So we can specify only those proeprties which we need, and .Net deserializer will ignore all other properties.

// { name: "", countrycode: "", population: -1 }
[DataContract]
public class City
{
	[DataMember(Name = "name")]
	public string Name { get; set; }

	[DataMember(Name = "countrycode")]
	public string CountryCode { get; set; }

	[DataMember(Name = "population")]
	public int Population { get; set; }
}

So now all that you need is to create an instance of the HttpWebRequest class, get the response and deserialize it by using the DataContractJsonSerializer class. The deserializer requires the System.Servicemodel.Web and System.Runtime.Serialization references.

You must use the two following methods to perform loading and parsing of JSON.

private void BeginDownloadCitiesRequest()
{
	// create the http request
	HttpWebRequest httpWebRequest = WebRequest.CreateHttp("http://api.geonames.org/citiesJSON?north=44.1&south=-9.9&east=-22.4&west=55.2&lang=en&username=vortexwolf");
	httpWebRequest.Method = "GET";
	httpWebRequest.Accept = "application/json";

	// get the response asynchronously
	httpWebRequest.BeginGetResponse(OnGetResponseCompleted, httpWebRequest);
}

private void OnGetResponseCompleted(IAsyncResult ar)
{
	var httpWebRequest = (HttpWebRequest)ar.AsyncState;

	// get the response
	var response = httpWebRequest.EndGetResponse(ar);

	// deserialize json
	var jsonSerializer = new DataContractJsonSerializer(typeof(CitiesList));
	var responseObject = (CitiesList)jsonSerializer.ReadObject(response.GetResponseStream());

	// display on the view
	Deployment.Current.Dispatcher.BeginInvoke(() => OnCitiesDownloaded(responseObject));
}

The code of creating a http request and getting a response will often repeat in different places of the application, so it is preferrable to move this code to a separate class.
I did it so:

public class HttpGetTask<T>
{
    public HttpGetTask(string url, Action<T> onPostExecute)
    {
        this.Url = url;
        this.OnPostExecute = onPostExecute;
    }

    public void Execute()
    {
        if (this.OnPreExecute != null)
        {
            this.OnPreExecute();
        }

        // create the http request
        HttpWebRequest httpWebRequest = WebRequest.CreateHttp(this.Url);
        httpWebRequest.Method = "GET";
        httpWebRequest.Accept = "application/json";

        // get the response asynchronously
        httpWebRequest.BeginGetResponse(OnGetResponseCompleted, httpWebRequest);
    }

    private void OnGetResponseCompleted(IAsyncResult ar)
    {
        var httpWebRequest = (HttpWebRequest)ar.AsyncState;

        // get the response
        HttpWebResponse response;
        try
        {
            response = (HttpWebResponse)httpWebRequest.EndGetResponse(ar);
        }
        catch (WebException)
        {
            this.InvokeOnErrorHandler("Unable to connect to the web page.");
            return;
        }
        catch (Exception e)
        {
            this.InvokeOnErrorHandler(e.Message);
            return;
        }

        if (response.StatusCode != HttpStatusCode.OK)
        {
            this.InvokeOnErrorHandler((int)response.StatusCode + " " + response.StatusDescription);
            return;
        }

        // response stream
        var stream = response.GetResponseStream();

        // deserialize json
        var jsonSerializer = new DataContractJsonSerializer(typeof(T));
        var responseObject = (T)jsonSerializer.ReadObject(stream);

        // call the virtual method
        this.InvokeInUiThread(() => this.OnPostExecute(responseObject));
    }

    private void InvokeOnErrorHandler(string message)
    {
        if (this.OnError != null)
        {
            this.InvokeInUiThread(() => this.OnError(message));
        }
    }

    private void InvokeInUiThread(Action action)
    {
        Deployment.Current.Dispatcher.BeginInvoke(action);
    }

    public string Url { get; private set; }

    public Action<T> OnPostExecute { get; private set; }

    public Action OnPreExecute { get; set; }

    public Action<string> OnError { get; set; }
}

You can execute this task by specifying Url and the OnPostExecute callback. Also you can set other properties. Example:

public class MainViewModel : INotifyPropertyChanged
{
    public MainViewModel()
    {
        Cities = new ObservableCollection<CityViewModel>();

        string url = "http://api.geonames.orgzz/citiesJSON?north=44.1&south=-9.9&east=-22.4&west=55.2&lang=en&username=vortexwolf";
        
        var task = new HttpGetTask<CitiesList>(url, this.OnPostExecute);
        task.OnPreExecute = this.OnPreExecute;
        task.OnError = this.OnError;

        task.Execute();
    }

    private void OnPreExecute()
    {
        this.IsLoading = true;
    }

    private void OnPostExecute(CitiesList responseObject)
    {
        this.OnCitiesDownloaded(responseObject);
        this.IsLoading = false;
    }

    private void OnError(string message)
    {
        MessageBox.Show(message);
        this.IsLoading = false;
    }

    public ObservableCollection<CityViewModel> Cities { get; set; }

    private bool _isLoading;

    public bool IsLoading
    {
        get { return _isLoading; }
        set
        {
            _isLoading = value;
            RaisePropertyChanged("IsLoading");
        }
    }

    private void OnCitiesDownloaded(CitiesList citiesList)
    {
        var cityModels = citiesList.Cities
            .Select(c =>
                    new CityViewModel
                    {
                        Name = c.Name,
                        CountryCode = c.CountryCode,
                        Population = c.Population
                    })
            .ToList();

        cityModels.ForEach(this.Cities.Add);
    }

    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    public void RaisePropertyChanged(string propertyName)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

I created the sample application for Windows Phone 7 which retrieves JSON data and displays it on the phone.

The source code of the example you can download here: PhoneJsonTest.zip