C#

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
using System.IO;

namespace DirectoryTreeView
{
	/// <summary>
	/// Description of MainForm.
	/// </summary>
	public partial class MainForm : Form
	{				
		public MainForm()
		{
			//
			// The InitializeComponent() call is required for Windows Forms designer support.
			//
			InitializeComponent();
			
			//
			// TODO: Add constructor code after the InitializeComponent() call.
			//
						
		}
						
		void FolderTreeViewBeforeExpand(object sender, TreeViewCancelEventArgs e)
		{
            MainForm.ActiveForm.Cursor = System.Windows.Forms.Cursors.WaitCursor;
        	TreeNode currentnode= e.Node;
        	if (currentnode.Nodes.Count==1) currentnode.Nodes[0].Remove();
            
            if (currentnode.Nodes.Count==0)
            {
	            int step = 0;
	            DirectoryInfo di = new DirectoryInfo(currentnode.FullPath);
	            DirectoryInfo[] dirs = null;
	            try
	            {
	            	dirs = di.GetDirectories();
	            }
	            catch(Exception ex)
	            {	                
	            }
	            
	            foreach (DirectoryInfo dir in dirs)
	            {
	                currentnode.Nodes.Add(dir.Name);
	                DirectoryInfo[] subfolder;
	                
	                try
	                {
	                    subfolder = dir.GetDirectories();
	                }
	                catch (Exception ex)
	                {
	                    continue;
	                }
	                if (subfolder.Length > 0)
	                {
	                    currentnode.Nodes[step].Nodes.Add("");
	                }
	                
	                step++;
	            }   
            }
            MainForm.ActiveForm.Cursor = System.Windows.Forms.Cursors.Default;
		}				
		
		void MainFormLoad(object sender, EventArgs e)
		{
			int step = 0;
            foreach(string str in Directory.GetLogicalDrives())
            {
                folderTreeView.Nodes.Add(str);                
                folderTreeView.Nodes[step].Nodes.Add("");
                step++;
            }	
		}
		
		void FolderTreeViewBeforeSelect(object sender, TreeViewCancelEventArgs e)
		{
			MainForm.ActiveForm.Text = e.Node.FullPath;
		}
		
		void FolderTreeViewAfterSelect(object sender, TreeViewEventArgs e)
		{
			MainForm.ActiveForm.Cursor=System.Windows.Forms.Cursors.WaitCursor;			
			thumbnailsFLP.Controls.Clear();
			displayPictureBox.Image = null;
			string[] Files = Directory.GetFiles(e.Node.FullPath);
			thumbnailsFLP.Controls.Clear();
						
			foreach (String fn in Files)
			{								
				PictureBox pic = new PictureBox();
				pic.SizeMode = PictureBoxSizeMode.StretchImage;
				
				if (fn.ToLower().EndsWith(".jpg") || fn.ToLower().EndsWith(".GIF") || fn.ToLower().EndsWith(".png") || fn.ToLower().EndsWith(".bmp") || fn.ToLower().EndsWith(".jpeg"))
				{
					pic.Image = Image.FromFile(fn);
					pic.Height = 100;
					pic.Width = 100;
					pic.Cursor = Cursors.Hand;
					thumbnailsFLP.Controls.Add(pic);
					pic.Click += new EventHandler(pic_Click);
				}
			}													
			MainForm.ActiveForm.Cursor=System.Windows.Forms.Cursors.Default;		
		}
		
		void pic_Click(object sender, EventArgs e)
		{
			PictureBox pic = (PictureBox)sender;
			displayPictureBox.SizeMode=PictureBoxSizeMode.StretchImage;
			displayPictureBox.Image = pic.Image;			
		}						
	}
}