WPFでTreeViewでデータバインドするサンプル２「エクスプローラーの左側」

using System.ComponentModel;
using System.Diagnostics;
using Reactive.Bindings;

namespace TreeViewSample01;

public class FolderItem : INotifyPropertyChanged
{
    // INotifyPropertyChanged
    public event PropertyChangedEventHandler? PropertyChanged;
    protected virtual void OnPropertyChanged(string name) =>
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    
    // コンストラクタ
    public FolderItem()
    {
        PropertyChanged += (s, e) => {};        
    }
    // フルパス
    public ReactiveProperty<string> FullPath { get; set; } = new("");
    // 名前
    public string Name
    {
        get
        {
            if ("" == FullPath.Value) return "PC";

            string name = System.IO.Path.GetFileName(FullPath.Value);
            return "" == name ? FullPath.Value.Substring(0, 2) : name;
        }
    }
    // サブフォルダ
    public ReactiveCollection<FolderItem> SubFolders { get; set; } = new();

    // 更新
    public void Update()
    {
        string path = FullPath.Value;
        if ("" == path)
        {
            var drives = System.IO.DriveInfo.GetDrives();
            foreach(var drive in drives)
            {
                try {
                    var attr = System.IO.File.GetAttributes(drive.Name);
                    var item = new FolderItem()
                    {
                        FullPath = new(drive.Name),
                    };
                    this.SubFolders.AddOnScheduler(item);
                } catch (Exception e)
                {
                    Debug.Print(e.Message);
                }

            }
        }
        else
        {
            var folders = System.IO.Directory.EnumerateDirectories(
                path, "*", System.IO.SearchOption.TopDirectoryOnly);


            foreach(var folder in folders)
            {
                var attr = System.IO.File.GetAttributes(folder);

                if ((attr & System.IO.FileAttributes.Hidden) == System.IO.FileAttributes.Hidden) continue;

                var item = new FolderItem()
                {
                    FullPath = new(folder),
                };
                this.SubFolders.AddOnScheduler(item);
            }
        }
    }


}

<Window
x:Class="TreeViewSample01.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:TreeViewSample01"
mc:Ignorable="d"
Height="450"
Width="800"
FontSize="24"
xmlns:interactivity="clr-namespace:Reactive.Bindings.Interactivity;assembly=ReactiveProperty.WPF"
xmlns:i="clr-namespace:Microsoft.Xaml.Behaviors;assembly=Microsoft.Xaml.Behaviors">
  <Window.DataContext>
    <local:MainWindowViewModel />
  </Window.DataContext>
  <i:Interaction.Behaviors>
    <local:ViewModelCleanupBehavior />
  </i:Interaction.Behaviors>
  <Grid>
    <TreeView ItemsSource="{Binding Folders}"
              SelectedValuePath="FullPath">
      <i:Interaction.Triggers>
          <i:EventTrigger EventName="SelectedItemChanged">
              <interactivity:EventToReactiveCommand Command="{Binding SelectedItemChangedCommand}" />
          </i:EventTrigger>
      </i:Interaction.Triggers>
      <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding SubFolders}">
            <TextBlock Text="{Binding Name}" />
        </HierarchicalDataTemplate>
      </TreeView.ItemTemplate>
    </TreeView>
  </Grid>
</Window>

using System.Diagnostics;
using System.ComponentModel;
using Reactive.Bindings;
using Reactive.Bindings.Extensions;
using System.Reactive.Disposables;
using System.Reactive.Linq;
using System.Windows;

namespace TreeViewSample01;
public class MainWindowViewModel : INotifyPropertyChanged, IDisposable
{
    // INotifyPropertyChanged
    public event PropertyChangedEventHandler? PropertyChanged;
    protected virtual void OnPropertyChanged(string name) =>
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    // IDisposable
    private CompositeDisposable Disposable { get; } = [];

    /**************************************************************************
    * プロパティ
    **************************************************************************/
    public ReactiveCollection<FolderItem> Folders { get; set; } = [];
    public ReactiveCommand<RoutedPropertyChangedEventArgs<Object>> SelectedItemChangedCommand { get; }

    public MainWindowViewModel()
    {
        PropertyChanged += (s, e) => {};

        SelectedItemChangedCommand = new ReactiveCommand<RoutedPropertyChangedEventArgs<Object>>()
        .WithSubscribe<RoutedPropertyChangedEventArgs<Object>>(e =>
        {
            if (e is null || e.NewValue is null) return;

            var fi = (FolderItem) e.NewValue;

            if ("" == fi.FullPath.Value) return;
            if (0 == fi.SubFolders.Count)
            {
                fi.Update();
            }
        });

        var root = new FolderItem()
        {
            FullPath = new(""),
        };
        root.Update();
        Folders.AddOnScheduler(root);
    }
    public void Dispose()
    {
        Debug.WriteLine("Dispose()");
        Disposable.Dispose();
    }
}