如何避免以下方法中的InvalidOperationException

所以，我有一个"扫描"ftp服务器的方法。首先，它扫描根目录，确定哪个内容是文件，哪个内容是文件夹，并将它们添加到两个不同的集合（文件夹的字符串列表和文件的字符串、int类型字典）。毕竟它自称。该方法在if…else语句中运行，该语句检查列表的计数。如果为零，该方法将扫描根文件夹。否则，它应该将列表的第一个元素连接到ftp地址并检查该文件夹。问题来了。每次我尝试执行该方法时，在第二次运行时（根扫描结束后），它都会抛出InvalidOperation Exception，因为"集合已修改；枚举操作可能无法执行"。我该如何避免这种情况？

这是代码：

internal void ListFilesOnServer()
    {
        ArrayList files = new ArrayList();
        if (directories.Count == 0)
        {
            try
            {
                FtpWebRequest ftpwrq = (FtpWebRequest)WebRequest.Create(server);
                ftpwrq.Credentials = new NetworkCredential(user, passw);
                ftpwrq.Method = WebRequestMethods.Ftp.ListDirectory;
                ftpwrq.KeepAlive = false;
                FtpWebResponse fresponse = (FtpWebResponse)ftpwrq.GetResponse();
                StreamReader sr = new StreamReader(fresponse.GetResponseStream());
                string temp = "";
                while ((temp = sr.ReadLine()) != null)
                {
                    files.Add(temp);
                }
                temp = String.Empty;
                sr.Close();
                fresponse.Close();
                DirOrFile(files);
            }
            catch (Exception e)
            {
                MessageBox.Show(e.Message);
            }
        }
        else
        {
            foreach (string file in directories)
            {
                try
                {
                    FtpWebRequest ftpwrq = (FtpWebRequest)WebRequest.Create(server+"/"+file);
                    ftpwrq.Credentials = new NetworkCredential(user, passw);
                    ftpwrq.Method = WebRequestMethods.Ftp.ListDirectory;
                    ftpwrq.KeepAlive = false;
                    FtpWebResponse fresponse = (FtpWebResponse)ftpwrq.GetResponse();
                    StreamReader sr = new StreamReader(fresponse.GetResponseStream());
                    string temp = "";
                    while ((temp = sr.ReadLine()) != null)
                    {
                        files.Add(temp);
                    }
                    temp = String.Empty;
                    sr.Close();
                    fresponse.Close();
                    DirOrFile(files);
                }
                catch (Exception e)
                {
                    MessageBox.Show(e.Message);
                }
            }
        }
        level = 1;
        ListFilesOnServer();
    }

foreach (string file in directories.ToArray()) {
    ...
}

for (var i = 0; i < directories.Count; ++i) {
    ... code which might append to directories
}