Binding a WPF ComboBox to ObservableCollection

Question

I usea ComboBox that doesn't seem to update the SelectedItem.

<ComboBox ItemsSource="{Binding Path=CountryEntries}"
          DisplayMemberPath="Country"
          SelectedValuePath="Country"
          SelectedValue="{Binding Path=CountryValue}"
          SelectedItem="{Binding Path=SelectedCountry}"/>

The ItemsSource of ComboBox is bound to a property "CountryEntries" on ViewModelClassB.

public class ViewModelClassB : INotifyPropertyChanged
{


    private readonly CollectionView countryEntries = new CollectionView(ListOfCountry()); // Type of ListOfCountry() is ObservableCollection<ClassA>
    public CollectionView CountryEntries
    {
        get
        {
            return countryEntries;
        }
    }

    public ClassB SelectedCountry
    {
        get => selectedCountry;
        set
        {
            this.selectedCountry = value;
            OnPropertyChanged();
            OnPropertyChanged(nameof(CityView));
        }
    }
}


public class ClassA : INotifyPropertyChanged
{
    public string Country { get; set; }
    public string City { get; set; }
}


public class ClassB : INotifyPropertyChanged
{
    public string Pro1 { get; set; }
    public string Pro2 { get; set; }
    public string CountryValue{ get; set; }

}

I get the following error:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

SelectedCountry.get returned null.

I've checked my code several times and can't seem to find anything that I'm doing wrong.

Answer 1

MainWindow.xaml:

<Grid>  
        <ComboBox ItemsSource="{Binding Path=CountryEntries}" Width="200" Height="60"  
           DisplayMemberPath="Country"   
           SelectedValuePath="Country"  
           SelectedItem="{Binding Path=SelectedCountry,  Mode=TwoWay}"/>  
    </Grid>  

MainWindow.xaml.cs:

using System.Collections.ObjectModel;  
using System.ComponentModel;  
using System.Runtime.CompilerServices;  
using System.Windows;  
using System.Windows.Data;  

namespace ComboboxFilter  
{  
  public partial class MainWindow : Window  
  {  
    public MainWindow()  
    {  
      InitializeComponent();  
      DataContext=new ViewModelClassB();  
    }  
  }  
  public class ViewModelClassB : INotifyPropertyChanged  
  {  
    private ObservableCollection<ClassA> listOfCountry;  
    public ObservableCollection<ClassA> ListOfCountry  
    {  
      get  
      {  
        return listOfCountry;  
      }  
      set { listOfCountry=value; OnPropertyChanged("ListOfCountry");}  
    }  
    private readonly ICollectionView countryEntries ;   
    public ICollectionView CountryEntries  
    {  
      get  
      {  
        return countryEntries;  
      }  
    }  
    public ViewModelClassB()  
    {  
      listOfCountry=new ObservableCollection<ClassA>();  
      listOfCountry.Add(new ClassA() {  City="Beijing",  Country="China"});  
      listOfCountry.Add(new ClassA() {  City= "New York",  Country= "United States" });  
      listOfCountry.Add(new ClassA() {  City= "Paris ",  Country= "France" });  
      countryEntries=CollectionViewSource.GetDefaultView(listOfCountry);  
    }  
    private ClassB selectedCountry;  
    public ClassB SelectedCountry  
    {  
      get => selectedCountry;  
      set  
      {  
        this.selectedCountry = value;  
        OnPropertyChanged("SelectedCountry");  
        //OnPropertyChanged(nameof(CityView));  
      }  
    }  
    public event PropertyChangedEventHandler PropertyChanged;  
    internal void OnPropertyChanged([CallerMemberName] string propName = "") => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));  
  }  
  public class ClassA   
  {  
    public string Country { get; set; }  
    public string City { get; set; }  
  }  
  public class ClassB   
  {  
    public string Pro1 { get; set; }  
    public string Pro2 { get; set; }  
    public string CountryValue { get; set; }  
  }  
}  

The result:

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