Listbox deselected on drag start

When I start dragging all items get deselected. Do you have a suggestion how can I do this?

<ListBox x:Name="FileListBox" ... SelectionMode="Extended"  PreviewMouseMove="List_MouseMove" >

private void List_MouseMove(object sender, MouseEventArgs e)
{
  if (e.LeftButton == MouseButtonState.Pressed )
  {
    cList files = GetSelectedFiles();
    string dataFormat = DataFormats.FileDrop;
    DataObject dataObject = new DataObject("myFormat", files);
    DragDrop.DoDragDrop(FileListBox, dataObject, DragDropEffects.Copy);
  }
}