TreeView Control with tri-state Logic

protected override void OnAfterCheck(TreeViewEventArgs e) {
   base.OnAfterCheck(e);
   ThreeStateTreeNode node = e.Node as ThreeStateTreeNode;
   if(node == null) return;
   node.ThreeStateChecked = (ThreeState)(((int)(node.ThreeStateChecked) + 1) % 3);
}

public partial class MainForm : Form {
   public MainForm() {
      InitializeComponent();
      foreach(var item in System.IO.Directory.GetDirectories("c:\\")) {
         var nd = new TreeViewThreeState.ThreeStateTreeNode(item);
         _threeState.Nodes.Add(nd);
         try {
            foreach(var item2 in System.IO.Directory.GetDirectories(nd.FullPath)) {
              nd.Nodes.Add(new TreeViewThreeState.ThreeStateTreeNode(item2));
            }
         } catch(UnauthorizedAccessException) { }
      }
   }
}

public ThreeStateThreeView()
{
    base.CheckBoxes = true;
    _imageList = new ImageList();
    _imageList.ImageSize = new Size(16, 16);
    if (UxTheme.IsAppThemed)
    {
        ThemePart checkboxThemes = new ThemeInfo()["BUTTON"].Parts["CHECKBOX"];
        _imageList.Images.Add(Properties.Resources.Checked0, Color.White);
        SetThemedCheckboxImage(checkboxThemes, "UNCHECKEDNORMAL");
        SetThemedCheckboxImage(checkboxThemes, "CHECKEDNORMAL");
        SetThemedCheckboxImage(checkboxThemes, "MIXEDNORMAL");
    }
    else
    {
        _imageList.Images.Add(Properties.Resources.Checked0, Color.White);
        _imageList.Images.Add(Properties.Resources.Checked1, Color.White);
        _imageList.Images.Add(Properties.Resources.Checked2, Color.White);
        _imageList.Images.Add(Properties.Resources.Checked3, Color.White);
    }
}

private void SetThemedCheckboxImage(ThemePart checkboxThemes, String stateName)
{
    Rectangle r = new Rectangle(0, 0, 16, 16);
    Bitmap bmp;
    Graphics g;
    bmp = new Bitmap(16, 16);
    g = Graphics.FromImage(bmp);
    checkboxThemes.States[stateName].DrawBackground(g, r);
    _imageList.Images.Add(bmp);
}