执行拖放操作时出现异常

我有一个WinForms应用程序,可以在两个TreeViews之间进行拖放操作。

在某些时候,我想拒绝底层业务实现中的操作,所以我抛出了一个Exception。我可以在输出窗口中看到异常,但问题是我在UI中看不到它,它也不会崩溃。

private TreeView tvLeft;
private TreeView tvRight;
private Dictionary<string, int> dico = new Dictionary<string, int>();

void tvLeft_DragDrop(object sender, DragEventArgs e) {

  if (e.Data.GetDataPresent(typeof(TreeNode))) {

    var tnSource = (TreeNode) e.Data.GetData(typeof(TreeNode));
    var tnDestination = tvLeft.GetNodeAt(tvLeft.PointToClient(new Point(e.X, e.Y)));

    // if I drag-drop the same node twice, there sould be an Exception
    // since the key is already in the dictionary...
    // ...but I get no Exception in the UI, the Application.ThreadException
    // or Appomain.CurrentDomain.UnhandledException handlers
    dico.Add(tnSource.Name, (new Random()).Next());

  }

}