当添加TreeNode到TreeView线程时，UI被锁定

我想当treenode添加到treeview时，ui不锁定(窗体可以通过拖动移动…)。我用头，但它不工作。请告诉我如何或帮助我解决这个问题。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;
namespace WindowsFormsApplication1
{
 public partial class Form1 : Form
 {
 TreeView tree;
 TreeNode root;
 Button button;
 public Form1()
 {
 this.Name = "Form1";
 this.Text = "Form1";
 root = new TreeNode("Hello");
 tree = new TreeView();
 tree.Location = new Point(0, 0);
 tree.Size = new Size(this.Width, this.Height - 70);
 tree.Anchor = AnchorStyles.Top | AnchorStyles.Bottom;
 this.Controls.Add(tree);
 tree.Nodes.Add(root);
 button = new Button();
 button.Text = "Add nodes";
 button.Location = new Point(0, this.Height - 70);
 button.Size = new Size(this.Width, 30);
 button.Anchor = AnchorStyles.Bottom;
 this.Controls.Add(button);
 button.Click += new EventHandler(button_Click);
 }
 void button_Click(object sender, EventArgs e)
 {
 Thread t = new Thread(new ThreadStart(this.AddNode));
 t.Start();
 }
 void AddNode()
 {
 if (this.tree.InvokeRequired)
 {
 this.tree.BeginInvoke(new MethodInvoker(this.AddNodeInternal));
 }
 else
 {
 this.AddNodeInternal();
 }
 }
 void AddNodeInternal()
 {
 root.Collapse();
 root.Nodes.Clear();
 TreeNode[] nodesToAdd = new TreeNode[20000];
 for (int i = 0; i < 20000; i++)
 {
 System.Threading.Thread.Sleep(1);
 TreeNode node = new TreeNode("Node " + i.ToString());
 nodesToAdd[i] = node;
 }
 root.Nodes.AddRange(nodesToAdd);
 root.Expand();
 }
 delegate void AddNodeDelegate();
 }
}

tree.SuspendLayout();
// Do updates here
tree.ResumeLayout();