【解決方法】MVVM に続いてカスケード コンボ ボックスを適切に実装するにはどうすればよいですか?

を使用する必要があります CollectionViewSource[^] サブコレクションをフィルタリングします。 Microsoft には次の例があります。 方法: ビューでデータをフィルター処理する – WPF .NET Framework | マイクロソフト ラーン[^]

以下は、MVVM パターンを使用して実装する方法を示すためにまとめられた実際の例です。

1. データソース提供: 世界の主要都市 – データセット – DataHub – 摩擦のないデータ[^]

2. 分離コード:

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Runtime.CompilerServices;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Data;

namespace WpfCascadingComboBoxes;

public partial class MainWindow : INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        _ = LoadData();
    }

    private string _url = "https://pkgstore.datahub.io/core/world-cities/world-cities_json/data/5b3dd46ad10990bca47b04b4739a02ba/world-cities_json.json";

    private CountryModel? selectedCountry;
    private ListCollectionView filteredCitiesView;

    public List<CityModel> Cities { get; } = new();
    
    public ObservableCollection<CountryModel> Countries { get; } = new();

    public ListCollectionView FilteredCitiesView
    {
        get => filteredCitiesView;
        private set
        {
            if (Equals(value, filteredCitiesView)) return;

            filteredCitiesView = value;
            OnPropertyChanged();
        }
    }

    public CountryModel? SelectedCountry
    {
        get => selectedCountry;
        set
        {
            if (Equals(value, selectedCountry)) return;

            selectedCountry = value;
            ApplyFilter();
            OnPropertyChanged();
        }
    }

    private async Task LoadData()
    {
        JsonSerializerOptions options = new();

        await using Stream stream = await new HttpClient().GetStreamAsync(_url);

        // deserialize the stream an object at a time...
        await foreach (CountryDTO item in JsonSerializer
                           .DeserializeAsyncEnumerable<CountryDTO>(stream, options))
        {
            Process(item);
        }

        PrepareFiltering();

        SelectedCountry = Countries.First();

        ApplyFilter();
    }

    private void PrepareFiltering()
    {
        FilteredCitiesView = (ListCollectionView)CollectionViewSource.GetDefaultView(Cities);
        FilteredCitiesView.SortDescriptions.Clear();
        FilteredCitiesView.SortDescriptions.Add(new SortDescription("Name",
            ListSortDirection.Ascending));
    }

    // Filter
    public bool Contains(object item)
    {
        CityModel? city = item as CityModel;
        return (city?.ParentId ?? -999) == (SelectedCountry?.Id ?? -1);
    }

    private void ApplyFilter()
        => FilteredCitiesView.Filter = Contains;

    // prepared models from JSON data
    private void Process(CountryDTO dto)
    {
        CountryModel? country = Countries
            .FirstOrDefault(x => x.Name.Equals(dto.Country));

        if (country == null)
        {
            country = new CountryModel(Countries.Count + 1, dto.Country);
            Countries.Add(country);
        }

        var city = new CityModel(Cities.Count + 1, country.Id, dto.Name);
        Cities.Add(city);
    }

    public event PropertyChangedEventHandler? PropertyChanged;

    protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
        => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

public class CountryDTO
{
    [JsonPropertyName("country")]
    public string? Country { get; set; }

    [JsonPropertyName("name")]
    public string? Name { get; set; }
}

public record CountryModel(int Id, string Name);

public record CityModel(int Id, int ParentId, string Name);

3. XAML:

<Window x:Class="WpfCascadingComboBoxes.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfCascadingComboBoxes"
        x:Name="Window"
        mc:Ignorable="d" Title="MainWindow" Height="450" Width="800">
    <Grid DataContext="{Binding ElementName=Window}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>

        <ComboBox Grid.Column="0"
                  Width="200"
                  VerticalAlignment="Center"
                  ItemsSource="{Binding Countries}"
                  DisplayMemberPath="Name"
                  SelectedItem="{Binding SelectedCountry}"/>

        <ComboBox Grid.Column="1"
                  Width="200"
                  VerticalAlignment="Center"
                  DisplayMemberPath="Name"
                  ItemsSource="{Binding FilteredCitiesView}"/>
    </Grid>
</Window>

ノート：
※アプリ起動時にデータを取り込み準備中です。 選択する前に、国の ComboBox がいっぱいになるのを待ちます。
* 国を選択すると、City ComboBox が CollectionViewSource[^].