TBaseVirtualTreeHelper = class helper for TBaseVirtualTree
  public
    function Get<T: class>(Node: PVirtualNode): T;
  end;

{ TBaseVirtualTreeHelper }

function TBaseVirtualTreeHelper.Get<T>(Node: PVirtualNode): T;
var
  ResultData: Pointer;
begin
  ResultData := GetNodeData(Node);
  if Assigned(ResultData) and (TObject(ResultData^) is T) then
    Result := T(ResultData^)
  else
    Result := nil;
end;

procedure TfrmDirectoryBrowser.vstDirectoryBrowserExpanding(Sender: TBaseVirtualTree; Node: PVirtualNode;
  var Allowed: Boolean);
var
  NodeData: TDirectory;
begin
  Allowed := True;

  NodeData := Sender.Get<TDirectory>(Node);
  NodeData.LoadSubDirectories; // ggf. auf Assigned prüfen
end;