执行拖放操作时发生异常

我有一个WinForms应用程序,我在两个TreeView之间进行拖放操作。

例外情况到哪里去了?

下面是一些描述问题的代码:

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());

  }

}