GUI Windows Form C#

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using System;
using System.Drawing;
using System.IO;
using System.Windows.Forms;
   
class DirectoryTreeView: TreeView
{
     public DirectoryTreeView()
     {
          ImageList = new ImageList();
          ImageList.Images.Add(new Bitmap(GetType(), "FLOPPY.BMP"));
          ImageList.Images.Add(new Bitmap(GetType(), "FOLD.BMP"));
          ImageList.Images.Add(new Bitmap(GetType(), "OPENFOLD.BMP"));
          RefreshTree();
     }
     public void RefreshTree()
     {
          BeginUpdate();
          Nodes.Clear();
          string[] astrDrives = Directory.GetLogicalDrives();
   
          foreach (string str in astrDrives)
          {
               TreeNode tnDrive = new TreeNode(str, 0, 0);
               Nodes.Add(tnDrive);
               AddDirectories(tnDrive);
   
               if (str == "C:\\")
                    SelectedNode = tnDrive;
          }
          EndUpdate();
     }
     void AddDirectories(TreeNode tn)
     {
          tn.Nodes.Clear();
   
          string          strPath = tn.FullPath;
          DirectoryInfo   dirinfo = new DirectoryInfo(strPath);
          DirectoryInfo[] adirinfo;
   
          adirinfo = dirinfo.GetDirectories();
   
          foreach (DirectoryInfo di in adirinfo)
          {
               TreeNode tnDir = new TreeNode(di.Name, 1, 2);
               tn.Nodes.Add(tnDir);
          }
     }
     protected override void OnBeforeExpand(TreeViewCancelEventArgs tvcea)
     {
          base.OnBeforeExpand(tvcea);
   
          BeginUpdate();
   
          foreach (TreeNode tn in tvcea.Node.Nodes)
               AddDirectories(tn);
   
          EndUpdate();
     }
}
class DirectoriesAndFiles: Form
{
     DirectoryTreeView dirtree;
     Panel             panel;
     TreeNode          tnSelect;
   
     public static void Main()
     {
          Application.Run(new DirectoriesAndFiles());
     }
     public DirectoriesAndFiles()
     {
          Text = "Directories and Files";
          BackColor = SystemColors.Window;
          ForeColor = SystemColors.WindowText;
   
          panel = new Panel();
          panel.Parent = this;
          panel.Dock = DockStyle.Fill;
          panel.Paint += new PaintEventHandler(PanelOnPaint);
   
          Splitter split = new Splitter();
          split.Parent = this;
          split.Dock = DockStyle.Left;
          split.BackColor = SystemColors.Control;
   
          dirtree = new DirectoryTreeView();
          dirtree.Parent = this;
          dirtree.Dock = DockStyle.Left;
          dirtree.AfterSelect += new TreeViewEventHandler(DirectoryTreeViewOnAfterSelect);
   
          Menu = new MainMenu();
          Menu.MenuItems.Add("View");
   
          MenuItem mi = new MenuItem("Refresh", new EventHandler(MenuOnRefresh), Shortcut.F5);
          Menu.MenuItems[0].MenuItems.Add(mi);
     }
     void DirectoryTreeViewOnAfterSelect(object obj, TreeViewEventArgs tvea)
     {
          tnSelect = tvea.Node;
          panel.Invalidate();
     }
     void PanelOnPaint(object obj, PaintEventArgs pea)
     {
          if (tnSelect == null)
               return;
   
          Panel         panel     = (Panel) obj;
          Graphics      grfx      = pea.Graphics;
          DirectoryInfo dirinfo   = new DirectoryInfo(tnSelect.FullPath);
          FileInfo[]    afileinfo;
          Brush         brush     = new SolidBrush(panel.ForeColor);
          int           y         = 0;
   
          afileinfo = dirinfo.GetFiles();
          foreach (FileInfo fileinfo in afileinfo)
          {
               grfx.DrawString(fileinfo.Name, Font, brush, 0, y);
               y += Font.Height;
          }
     }
     void MenuOnRefresh(object obj, EventArgs ea)
     {
          dirtree.RefreshTree();
     }
}