try this
TreeViewItem Treeviewitem = (TreeViewItem)sender;
if (Treeviewitem.Items.Count == 1 && Treeviewitem.Items[0] == dummyNode)
{
Treeviewitem.Items.Clear();
try
{
foreach (string s in Directory.GetDirectories(Treeviewitem.Tag.ToString()))
{
TreeViewItem Treeviewsubitem = new TreeViewItem();
Treeviewsubitem.Header = s.Substring(s.LastIndexOf("\\") + 1);
Treeviewsubitem.Tag = s;
Treeviewsubitem.FontWeight = FontWeights.Normal;
Treeviewsubitem.Items.Add(dummyNode);
Treeviewsubitem.Expanded += new RoutedEventHandler(folder_Expanded);
Treeviewitem.Items.Add(Treeviewsubitem);
}
}
catch (Exception) { }
}