FolderBrowserDialog folderBrowserDialog = new FolderBrowserDialog();
            
            if(folderBrowserDialog.ShowDialog(this) == DialogResult.OK)
            {
                fullDirectoryName = folderBrowserDialog.SelectedPath;
                directoryName = Path.GetDirectoryName(fullDirectoryName);
                this.txt_DireName.Text = fullDirectoryName;
            }

this.txt_DireName.Text = new System.IO.DirectoryInfo(folderBrowserDialog.SelectedPath).Name;

------解决方案--------------------
DirectoryInfo 类的操作