一个线程的lock应该如何写！

 public void SaveUrlPics(string url, string path, Random ran)        {            WebClient wc = new WebClient();                        string address = path + "\\" + CreateRandomCode(8) + GetSuffix(path);            try            {//在这里我想把当前线程等待                wc.DownloadFile(url, address);                wc.DownloadFileCompleted += new System.ComponentModel.AsyncCompletedEventHandler(wc_DownloadFileCompleted);            }            catch (Exception ex)            {                ListUrl.writeLog(ex.Message);            }            finally            {                wc.Dispose();            }        }        void wc_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)        {            //下载完成后，放走线程。        }