c# UWP binding ComboBox SelectedItem

Question

Hello I can't seem to figure out why my combox stays empty :/

On page load the first combox gets populated with countries (from JSON), the second combobox should populate when a country is selected in the first combobox. I'm trying to fetch the SelectedItem (country) as string in a property ... SelectedItem is of type ComboBoxItem ? I think that is where it goes wrong.

The (view)model where the sorting bindable properties are:

public class LocalityModel : NotifyProp
{
    #region properties
    private static List<LocalityJSON> dataList;
    public List<LocalityJSON> DataList
    {
        get
        {
                return dataList;
        }
        set {
            dataList = value;
            RaisePropertyChanged("Landen");
            RaisePropertyChanged("Gewesten");
        }
    }

    public List<string> Landen
    {
        get { if (DataList == null) return null; return (from s in DataList orderby s.Land select s.Land).Distinct().ToList<string>(); }
    }
    public string SelectedLand { get; set; }
    public List<string> Gewesten {
        get { if (DataList == null) return null; return (from s in DataList where s.Land.Equals(SelectedLand) select s.Gewest).Distinct().ToList<string>(); }
    }
    #endregion
    #region ctor
    public LocalityModel()
    {
        FillDataList();
    }
    #endregion
    #region methodes
    public async void FillDataList()
    {
        if (DataList == null)
        {
            DataList = await EVNT.Entries();
        }
    }
    #endregion
}

MainPage XAML (the bindings):

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}" DataContext="{Binding Source={StaticResource LocalityModel}}">
...
        <TextBlock x:Name="txbCountry" Style="{StaticResource InfoLabelCountry}" />
        <ComboBox x:Name="cboCountry" Style="{StaticResource CountryBox}" ItemsSource="{Binding Landen}" SelectedItem="{Binding SelectedLand, Mode=TwoWay}" />
        <TextBlock x:Name="txbGewest" Style="{StaticResource InfoLabelGewest}" />
        <ComboBox x:Name="cboGewest" Style="{StaticResource GewestBox}" ItemsSource="{Binding Gewesten}" />

INotifyPropertyChanged:

public class NotifyProp : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public void RaisePropertyChanged(string propertyName)
    {
        if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

The model for the JSON:

public class LocalityJSON
{
    public string FB_ID { get; set; }
    public string Land { get; set; }
    public string Gewest { get; set; }
    public string City { get; set; }
}

The JSON deserialisation (less important for the question):

public class EVNT
{
    public async static Task<List<LocalityJSON>> Entries()
    {
        using (var client = new HttpClient())
        {
            client.BaseAddress = new Uri(@"http://e-vnt.com/admin/core/api/");
            HttpResponseMessage response = await client.GetAsync("localityApi");
            if (response.IsSuccessStatusCode)
            {
                String s = await response.Content.ReadAsStringAsync();
                List<LocalityJSON> entries = JsonConvert.DeserializeObject<List<LocalityJSON>>(s);
                return entries;
            }
            else
                return null;
        }
    }
}

Answer 1

In your SelectedLand Property setter you need to fire PropertyChanged event the for both SelectedLand and for Gewesten.

It would probably look something like this

private string _SelectedLand;
public string SelectedLand
{
   get
   {
      return _SelectedLand;
   }
   set
   {
      _SelectedLand = value;
      RaisePropertyChanged("SelectedLand");
      RaisePropertyChanged("Gewesten");
   }
}

if you don't fire PropertyChanged event for Gewesten then that combobox will not know to reload is values.