为什么有些目录我不能在listBox1上拖放？

Question

这是一个文件夹的路径，里面有我想要拖放到listBox1中的图片：

C：\ Users \ chocolade1972 \ AppData \ Local \ mws \ My Weather Station \ images

但是，如果我在c：\中拖动一个文件夹，例如c：\ images，那么它将被精确拖动到listBox1中。

当我拖动用户...目录时，我看到这个黑色符号，我无法将其删除。

private void listBox1_DragDrop(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.FileDrop))
            {
                string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
                label6.Text = files.Length.ToString();
                filesCount = files.Length-1;
                beginConvertionToolStripMenuItem.Enabled = true;
                for (int i = 0; i < files.Length; i++)
                {
                    FileInfo fi = new FileInfo(files[i]);
                  if (listBox1.Items.Contains(fi.FullName))
                  {

                  }
                  else
                  {
                      string directoryName = fi.DirectoryName;
                      string[] filesExtension = Directory.GetFiles(directoryName).Select(p => Path.GetExtension(p)).Distinct().OrderBy(p => p).ToArray();
                      for (int d = 0; d < filesExtension.Length; d++)
                      {
                          if (fi.Extension.ToLower() == filesExtension[d])
                          {
                              listBox1.Items.Add(fi.FullName);
                          }
                      }

                  }
                }
                this.Select();
            }
        }

拖延事件：

private void listBox1_DragOver(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.FileDrop))
            {
                string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);

                bool bfound = false;
                for (int i = 0; i < files.Length; i++)
                {
                    FileInfo fi = new FileInfo(files[i]);
                    string directoryName = fi.DirectoryName;
                    string[] filesExtension = Directory.GetFiles(directoryName).Select(p => Path.GetExtension(p)).Distinct().OrderBy(p => p).ToArray();
                    for (int d = 0; d < filesExtension.Length; d++)
                      {
                          if (fi.Extension.ToLower() == filesExtension[d])
                        {
                            bfound = true;
                        }
                      }
                }

                if (bfound)
                    e.Effect = DragDropEffects.Copy;
                else
                    e.Effect = DragDropEffects.None;
            }
            else
                e.Effect = DragDropEffects.None;

        }