C# yield download callback

yield async.cs

//Xu Pei Yao - 2015/03/23 - 22:35
class Program  {
        static void DownloadHtml(string url,out string html,ref int time) {
            string result = new WebClient().DownloadString(url);
            lock(result) {
                time--;
                html = result;
                if(time == 0)th.MoveNext();//跳脫點繼續
            }
        }

        static IEnumerable DownloadPages(params string[] urls) {//迭代器
            string[] HtmlAry = new string[urls.Length];
            Thread[] DownloadThreads = new Thread[urls.Length];
            int time = urls.Length;//計數器
            for(int i = 0 ; i < urls.Length;i++) {
                DownloadThreads[i] = new Thread((obj) => {
                    DownloadHtml(urls[(int)obj] ,out HtmlAry[(int)obj] ,ref time);
                });
            }
            for(int i = 0 ; i < urls.Length;i++){
                DownloadThreads[i].Start(i);
            }

            yield return null;//跳脫

            foreach(string html in HtmlAry)
                Console.WriteLine(html.Length);
        }

        static IEnumerator th;//迭代器
        static void Main(string[] args) {
            Console.WriteLine("Main START");
            th = DownloadPages(//同時下載以下頁面，完成後顯示所有頁面的長度
                "http://www.google.com.tw/",
                "http://www.yahoo.com.tw/",
                "http://www.nkfust.edu.tw/"
            ).GetEnumerator();
            th.MoveNext();
            Console.WriteLine("Main End");
            
            Console.ReadKey();
        }
    }