This is a MSDN question asked by WPF tree view how do I add to the selected item , the source is WPF tree view how do I add to the selected item .

        public List<TreeViewList> treelist = new List<TreeViewList>();
        private void AddFolder_Click(object sender, RoutedEventArgs e)
            if (list.SelectedItem != null) 
                    var listof = (list.ItemsSource as List<TreeViewList>);
                    int curIndex = listof.IndexOf(list.SelectedItem as TreeViewList);
                    listof[curIndex].Children.Add(person);
                catch 
                    var listof = (list.ItemsSource as List<TreeViewList>);
                    for (int a = 0; a < listof.Count; a++) 
                        for (int b = 0; b < listof[a].Children.Count; b++)
                            if (list.SelectedItem == listof[a].Children[b]) 
                                listof[a].Children[b].Children.Add(person);
        public class TreeViewList
            public string Name { get; set; }
            public TreeViewList()
                Children = new ObservableCollection<TreeViewList>();   
            public ObservableCollection<TreeViewList> Children { get; set; }      
 use ObservableCollection instead of list,  
 use SelectedItemChanged event to get SelectedItem.  
Try following demo. Demo use Interaction and MVVM.  
XAML:  
<Window x:Class="WpfApp1.Window13"
        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:WpfApp13"
        xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
        mc:Ignorable="d"
        Title="Window13" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <StackPanel>
    <Button Content="Add new person" Command="{Binding Cmd}"/>
    <TreeView ItemsSource="{Binding View}" Margin="5">
      <i:Interaction.Behaviors>
        <local:TreeViewBehavior/>
      </i:Interaction.Behaviors>
      <TreeView.Resources>
        <HierarchicalDataTemplate DataType="{x:Type local:TreeViewList}"
                                  ItemsSource="{Binding Children}">
          <Label Content="{Binding Name}"/>
        </HierarchicalDataTemplate>
      </TreeView.Resources>
    </TreeView>
  </StackPanel>
</Window>
Code:  
using System;
using System.Collections.ObjectModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interactivity;
namespace WpfApp13
  public class ViewModel
    public ObservableCollection<TreeViewList> treelist = new ObservableCollection<TreeViewList>();
    public ObservableCollection<TreeViewList> View { get => treelist; }
    public TreeViewList SelectedItem { get; set; }
    public ICommand Cmd { get => new RelayCommand(AddFolder_Click); }
    private int nr = 1;
    private TreeViewList person { get => new TreeViewList() { Name = $"Name {nr++}" }; }
    private void AddFolder_Click(object state)
      if (SelectedItem == null) treelist.Add(person); else SelectedItem.Children.Add(person);
  public class TreeViewList
    public string Name { get; set; }
    public ObservableCollection<TreeViewList> Children { get; set; } = new ObservableCollection<TreeViewList>();
  public class TreeViewBehavior : Behavior<TreeView>
    protected override void OnAttached()
      AssociatedObject.SelectedItemChanged += AssociatedObject_SelectedItemChanged;
    private void AssociatedObject_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
      var tv = sender as TreeView;
      if (tv == null) return;
      var vm = tv.DataContext as ViewModel;
      if (vm == null) return;
      vm.SelectedItem = tv.SelectedItem as TreeViewList;
  public class RelayCommand : ICommand
    private readonly Predicate<object> _canExecute;
    private readonly Action<object> _action;
    public RelayCommand(Action<object> action) { _action = action; _canExecute = null; }
    public void Execute(object o) => _action(o);
    public bool CanExecute(object o) => _canExecute == null ? true : _canExecute(o);
    public event EventHandler CanExecuteChanged
      add { CommandManager.RequerySuggested += value; }
      remove { CommandManager.RequerySuggested -= value; }
Thanks.