question

Hi, try following demo:

 <Window x:Class="WpfApp1.Window48"
         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:WpfApp48"
         mc:Ignorable="d"
         Title="Window48" Height="450" Width="800">
   <Window.DataContext>
     <local:ViewModel/>
   </Window.DataContext>
   <Grid>
     <TreeView ItemsSource="{Binding View}">
       <TreeView.Resources>
         <HierarchicalDataTemplate DataType="{x:Type local:Folder}" ItemsSource="{Binding SubFolders}">
           <TextBlock Text="{Binding Name}"/>
         </HierarchicalDataTemplate>
       </TreeView.Resources>
     </TreeView>
   </Grid>
 </Window>

 using System.Collections.ObjectModel;
 using System.ComponentModel;
 using System.IO;
 using System.Windows;
 using System.Windows.Data;
 using IO = System.IO;
    
 namespace WpfApp48
 {
   public class ViewModel
   {
     public ViewModel()
     {
       string path = @"c:\temp";
       foreach (var dir in Directory.GetDirectories(path)) LoadFolder(dir, col);
       cvs.Source = col;
     }
    
     private void LoadFolder(string path, ObservableCollection<Folder> col)
     {
       Folder folder = new Folder() { Name = IO.Path.GetFileNameWithoutExtension(path) };
       col.Add(folder);
       foreach (var dir in Directory.GetDirectories(path)) LoadFolder(dir, folder.SubFolders);
     }
     public ICollectionView View { get => cvs.View; }
     private CollectionViewSource cvs = new CollectionViewSource();
     private ObservableCollection<Folder> col = new ObservableCollection<Folder>();
   }
    
   public class Folder
   {
     public string Name { get; set; }
     public ObservableCollection<Folder> SubFolders { get; set; } = new ObservableCollection<Folder>();
   }
 }