WPF

<Window x:Class="Tree.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Tree"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TreeView Name="tvTree">
            <TreeView.ItemTemplate>
                <HierarchicalDataTemplate ItemsSource="{Binding SubNodes}" DataType="{x:Type local:Node}">
                    <Grid>
                        <TextBlock Text="{Binding Header}" />
                    </Grid>
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>
        <Button Content="Button" Height="50" HorizontalAlignment="Left" Margin="456,263,0,0" Name="button1" VerticalAlignment="Top" Width="47" Click="button1_Click" />
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Tree
{
    public partial class MainWindow : Window
    {
        Node top;
        Node root;

        public MainWindow()
        {
            InitializeComponent();
                        
            root = new Node();
            root.Header = "Root";

            root.SubNodes.Add(new Node("B1"));

            top = new Node();
            top.SubNodes.Add(root);

            Update();
        }

        public void Update()
        {
            tvTree.ItemsSource = null;
            tvTree.ItemsSource = top.SubNodes;
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            root.SubNodes.Add(new Node("B2"));
            Update();
        }
    }

    public class Node
    {
        List<Node> subNodes = new List<Node>();
        string header;

        public Node()
        {

        }

        public Node(string header)
        {
            this.header = header;
        }

        public string Header
        {
            get { return header; }
            set { header = value; }
        }

        public List<Node> SubNodes
        {
            get { return subNodes; }
            set { subNodes = value; }
        }
    }
}